Why was Thranduil not given one of the Three Elven Rings?

Question

The three Rings of Power made for the Elves are borne by Galadriel, Elrond, and Círdan. Those Rings are described as:

Three Rings for the Elven-kings under the sky,
Seven for the Dwarf-lords in their halls of stone,
Nine for Mortal Men doomed to die,
One for the Dark Lord on his dark throne
In the Land of Mordor where the Shadows lie.

None of the three bearers during the War of the Ring is a king.

In contrast, Thranduil is a king and one with a notably large kingdom. So why wasn’t Thranduil given one of the Three?

Answer 1

Galadriel, Elrond (and by extension Gil-Galad) and Círdan were all Elves of greater stature than Thranduil.

• Gil-Galad (and technically Elrond after him) was the High King of the Noldor.
• Elrond, although not taking the crown, was the rightful High King of the Noldor after Gil-Galad, one of the wisest Elves and Lord of Rivendell and brother of the King of Númenor.
• Galadriel was born in the light of the Trees and was the (half-)niece of Fëanor, one of the greatest Elves of all time, and of the House of Finarfin. Making the crossing of the Helcaraxë with the Noldorin Exiles.
• Círdan was born at Cuiviénen¹, the waking place of the Elves, and is one of the oldest Elves alive and Master of the Havens. An incredibly important role, in which he sends Elves back to Valinor.

Thranduil, while a king, was a king of Silvan Elves, a group of Elves that gave up the journey to Valinor quite early. Although they were Nandor (and therfore Teleri) in origin. In terms of the grandeur of the Elves, Thranduil was a far less important Elf-lord(/Lady) in comparison to the other three.

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¹ Given the incredibly small amount of detail ever given about Círdan it may not be a surprise that it's never explicitly stated that Círdan was born at Cuiviénen. However, I think the evidence for it makes it pretty conclusive. Círdan is described as kin of Elwë, having the same silvery hair:

Elwë himself had indeed long and beautiful hair of silver hue, but this does not seem to have been a common feature of the Sindar, though it was found among them occasionally, especially in the nearer or remoter kin of Elwë (as in the case of Círdan)
_{The War of the Jewels, Part IV: Quendi and Eldar}

Círdan was a Telerin Elf, one of the highest of those who were not transported to Valinor but became known as the Sindar, the Grey-elves; he was akin to Olwë, one of the two kings of the Teleri, and lord of those who departed over the Great Sea. He was thus also akin to Elwë, Olwë's elder brother, acknowledged as high-king of all the Teleri in Beleriand, even after he withdrew to the guarded realm of Doriath.
_{The People's of Middle-earth, Part Two: Late Writings, XIII: Late Writings}

Círdan also became a leader of his people after the loss of Elwë:

Nonetheless it is said that for love of his kin and allegiance Círdan was the leader of those who sought longest for Elwë when he was lost and did not come to the shores to depart from Middle-earth. Thus he forfeited the fulfilment of his greatest desire: to see the Blessed Realm and find again there Olwë and his own nearest kin. Alas, he did not reach the shores until nearly all the Teleri of Olwë's following had departed.
_ibid.

From the Annals of Aman in Morgoth's Ring, we get the following dates for various events. Primarily the departure of the Elves on the Great Journey from Cuiviénen at 1105 Y.T (Year of the Trees, each of which is 10 years of the Sun). 25 years later, in 1130 Y.T. Elwë, the King of the Teleri was lost and is when Círdan is described as becoming the leader of those that didn't follow Olwë and was thus the third highest leader of the Teleri. Given he was raised to that position 25 years after they left Cuiviénen (250 years of the sun), I find it highly unlikely he was born on the Great Journey and not in Cuiviénen, and wasn't amongst those that were born at Cuiviénen.

Answer 2

So why wasn't Thranduil given a Ring?

He wasn't tight with Celebrimbor, who made the rings; and he didn't do anything to merit Celebrimbor giving him one.

And about him being king... "King of the who?"

Answer 3

There aren't just "elves", there are many different kinds of elves. They are different elven people - or races if you will, since they tend to have different hair and eye colors depending on ancestry.

Some of these are considered of higher birth than others, depending on how far they came in their travels to Valinor. It's complicated, see https://lotr.fandom.com/wiki/Teleri?file=Sundering_of_the_Elves.jpg. The elven people Vanyar, Noldor and partially Teleri went to live in Valinor and those are referred to as Calaquendi the "light elves" or "high elves". It's kind of elitist/racist but generally you have "the finest" from left to right:
Vanyar > Noldor > Teleri (> Sindar > Nandor) > the rest.

Each of these have a High King or in some cases many Kings. They might in some cases also have one King in Valinor and one in Middle-Earth.

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The Three rings were forged by Celebrimbor of Gwaith-i-Mírdain, a small Noldor elf nation who descend from the House of Fëanor. Celebrimbor was the last member of this House. It wouldn't make any sense for him to give the rings to elves who weren't Noldor nobility.

The verse "Three Rings for the Elven-kings" would supposedly have been written during the time when the Three rings were forged. And during that time, the title High King of Noldor belonged to Gil-Galad, so the most natural choise was to give the rings to him. And Celebrimbor did give two of the rings to him. The third was given to Galadriel, who is also high nobility among Noldor, the highest ranked member of the House of Finarfin in Middle Earth. Galadriel was Celebrimbor's father's cousin and Gil-Galad was his second cousin.

So the "elven kings" referred to in the poem were possibly Celebrimbor himself(?), Gil-Galad and Galadriel. These were the Noldor elves of highest standing but also the highest ranked members of the houses of Fëanor, Fingolfin and Finarfin respectively. And also Celebrimbor's closest relatives.

Gil-Galad in turn passed his rings to Elrond and Círdan who both served Gil-Galad during the second age. Elrond was also Gil-Galad's closest relative and heir. Círdan was not of Noldor but a Teleri/Sindar noble. Círdan's lineage is supposedly from the Teleri Kings, Thingol and Olwë (which would make him a distant relative to Galadriel on her mother's side). He also fought in a lot of the great wars during the first and second ages so he had earned some renown. Círdan later passed on his ring to Gandalf.

Thranduil is a Sindar noble of unclear descent and he is King over Nandor elves. He has no connection to Celebrimbor or Noldor at all.

Answer 4

Expanding on Edlothiad's detailed answer, it should also be noted that the bearers of the Elven rings were all, at a time or another, members of the White Council, while Thranduil was not.

[...] and in that time was first made the Council of the Wise that is called the White Council, and therein were Elrond and Galadriel and Círdan, and other lords of the Eldar, and with them were Mithrandir and Curunír. And Curunír (that was Saruman the White) was chosen to be their chief, for he had most studied the devices of Sauron of old. Galadriel indeed had wished that Mithrandir should be the Lead of the Council, and Saruman begrudged them that, for his pride and desire of mastery was grown great; but Mithrandir refused the office, since he would have no ties and no allegiance, save to those who sent him, and he would abide in no place nor be subject to any summons. But Saruman now began to study the lore of the Rings of Power, their making and their history.
The Silmarillion, "Of the Rings of Power and the Third Age"

While the text reports that in this first meeting of the White Council there were indeed "other lords of the Eldar", it seems clear that the most important of them all were Elrond, Galadriel and Cirdan, who ruled the three most important realms of the elves in Middle-earth, respectively Imladris (Rivendell), Lothlorien and Mithlond (the Grey Havens).

Compared to these three locations, the Woodland Realm ruled by Thranduil was far less important, and both him and his people were rather isolationist and far less involved in the great events of the Second and Third Age of Middle-earth, having a notable part only on the Quest of Erebor and the Battle of the Five Armies (both of them described in "The Hobbit").

It is possible, and in my opinion, very likely, that Thranduil was present during this meeting as one of the "other lords of the Eldar", but his stature is not high enough to be explicitly mentioned by name; Elrond, Galadriel and Cirdan instead are explicitly named, and they are the first bearers of the Elven Rings.
Besides Elves, the other most important members of the White Council were Gandalf (Mithrandir) and Saruman (Curunir): the stature of Gandalf is further confirmed by Cirdan giving him his Ring, when he arrived in Middle-earth, so it passed from one of the most important Elves to one of the most important Istari. All of them had a large and active part in the struggle against Sauron, while Thranduil was king of a regional power at best and far less prominent in the grand strategical layout.

It should also be noted that the Rings were not mere ornaments or symbols of regal status, they were effectively instruments to be used in the war against Sauron, even if not directory in battle they had a power that their bearer could actively use to pursue their goals. This is the reason why Cirdan gave Narya to Gandalf (it could better serve the cause in the hands of the Wizard) and why the other two were in the hands of the rulers of the most important realms of the Elves, not just from a prestige point of view but for strategical reasons as well.

In this respect, the verse that literally reads "Elven-kings", should be meant as a more generic Elven lords (and it is curious that the "Dwarf-lords" who received the Seven rings were instead titled Kings of their own realms).