Out of all the ancient blades made by Elvish weapon-smiths in Gondolin, why was Sting given a distinctive and unique glow whenever an Orc was close by?
Secondly, why would they put it on such a relatively small weapon, as opposed to Orcrist or Glamdring?
Orcrist and Glamdring were supposed to.
"being the work of Elvish smiths in the Elder Days these swords shone with a cold light, if any Orcs were near at hand,"
In the Fellowship of the Ring (the novel), Glamdring glowed with a "pale-light".
From The Hobbit (the novel):
They buried Thorin deep beneath the Mountain... Upon his tomb the Elvenking then laid Orcrist, the elvish sword that had been taken from Thorin in captivity. It is said in songs that it gleamed ever in the dark if foes approached, and the fortress of the dwarves could not be taken by surprise.
Peter Jackson, in the Fellowship of the Rings (EE) commentary, upon being asked why Glamdring didn't glow in the Mines of Moria, responded with 'budgetary cuts?' and something about 'running out of blue'- as such, it seems that it was an oversight.
All 3 blades should glow; the Hobbit informs us that they were made in Gondolin for the goblin wars, so let's see what the Silmarillion has to say (Chapter 15, "of the Noldor in Beleriand").
So, it was prophecied that Gondolin would last longest of all Elven strongholds:
Ulmo came to him once again, and spoke with him. And he said: "....Longest of all the realms of the Eldalie shall Gondolin stand against Melkor."
But also that:
And Ulmo warned Turgon that he also lay under the Doom of Mandos, which Ulmo had no power to remove. "Thus it may come to pass," he said, "that the curse of the Noldor shall find thee too ere the end, and treason awake within thy walls. Then they shall be in peril of fire."
I believe that nothing more need be said. With that kind of doom hanging over you - it's useful to know when Orcs are nearby.
It glowed blue because it was supposed to as it was made in Gondolin just as Orcrist and Glamdring were as well. The swords were Gandalf and Thorin were using improperly did not glow because Peter Jackson could not make them glow blue without making them look like light sabers from Star Wars. This is the only reason. The movies are not an accurate representation of Tolkien's work. They were the best that Jackson could do with the money and technology available and the time constraints involved including how long the movies played.
Every sword made in Gondolin should indeed glow blue and the elves of Gondolin were not worried that the goblins would see them coming. They wanted them to know it, because goblins are cowardly creatures and would be wracked with fear before the elves even got to them. Please do not use the Peter Jackson adaptations of Tolkien's work as your reference material for any subject. They were great movies but massively limited in their scope as Tolkien's epic saga could never be told in something as simple as a trilogy. You would need at least six movies if not more to make even an attempt at bringing the books truly to life.
Just to reiterate because it needs to be; never, ever use Peter Jackson's movies to explain anything at all that has to do with Tolkien's reasoning, decisions, or choices in how he wrote the books and what he wrote in them.
I was disappointed that Glamdring and Orcrist didn't glow in The Hobbit, but I understand IF, for the same of continuity, Jackson chose not to let them glow.
I like to think it was an intentional and TACTICAL choice by the Gondolin elves to make only small weapons like knives [Sting] glow. Imagine trying to sneak up on an unwary orc or goblin in a dark mine or cave with a large sword like Orcrist glowing brightly like Sting does in The Hobbit.
