The Wikipedia article "Orc (Middle-earth)" informs,
The Hobbit generally uses the term goblin, while The Lord of the Rings prefers orc.
Furthermore, Andres F.'s answer "Are orcs and goblins really the same thing?" establishes,
For Tolkien, Goblin and Orc are two "translations" of the same word.
Therefore, since the terms goblin and orc are synonymous, why does J. R. R. Tolkien primarily refer to goblins in The Hobbit but uses the term orcs in The Lord of the Rings?
The Hobbit was written before The Lord of the Rings and as such wasn't originally part of the Legendarium, it was later added into the Legendarium after its success and the beginnings of the writings of its sequel, The Lord of the Rings and The Silmarillion. This change occurs in the second edition of The Hobbit where quite a few things change from the original. After The Hobbit, Tolkien decided 'Orc' was a better English translation for the Sindarin 'Orch' or Westron 'Orka' that his works were supposedly originally written in. Tolkien gives us a little insight into is preference in letter 151:
Your preference of goblins to orcs involves a large question and a matter of taste, and perhaps historical pedantry on my part. Personally I prefer Orcs (since these creatures are not 'goblins', not even the goblins of George MacDonald, which they do to some extent resemble).
The Letters of J. R. R.Tolkien — Letter 151, to Hugh Brogan, 18 September 1954
The letter above comes after the second edition of The Hobbit was published in 1951 when Tolkien changed The Hobbit so that it would fit into the mythology he was building and The Lord of the Rings. While its initial setting as a light, quick-paced children's book may have influenced the initial choice of words, the reason words such as Goblin become Orc and Necromancer become Sauron, is because of its introduction into the greater mythology that Tolkien began building decades before The Hobbit.
The word Orc does appear in The Hobbit, 2 times on its own and 6 times as "Orcrist". It is interesting to not that both times it's used on its own it seems to reference larger goblins. This is reminiscent of the original state of the story before its addition into the Legendarium.
not knowing that even the big ones, the orcs of the mountains...
Before you could get round Mirkwood in the North you would be right among the slopes of the Grey Mountains, and they are simply stiff with goblins, hobgoblins, and orcs of the worst description.
This, Thorin, the runes name Orcrist, the Goblin-cleaver in the ancient tongue of Gondolin
They had called it Orcrist, Goblin-cleaver, but the goblins called it simply Biter.
Orcrist, too, had been saved; for Gandalf had brought it along as well, snatching it from one of the terrified guards.
You nearly chopped off my head with Glamdring, and Thorin was stabbing here there and everywhere with Orcrist.
“We have none,” said Thorin, and it was true enough: their knives had been taken from them by the wood-elves, and the great sword Orcrist too.
Upon his tomb the Elvenking then laid Orcrist, the elvish sword that had been taken from Thorin in captivity.
The style of writing is very different from that of The Lord of the Rings.
One of those differences is that Tolkien uses fewer "foreign" words in The Hobbit. Presumably, his opinion at the time was that a large number of unfamiliar words would not be acceptable to his audience, especially an audience of children (an opinion that may have changed by the time he published The Lord of the Rings). Instead, he tends to use English words (that are sometimes translations, and sometimes not). For example in The Hobbit:
He also tends to provide immediate translations of the invented words that he does use. For example, when Elrond tells Gandalf and the Dwarves the history of the swords they got from the troll's lair, he says:
This, Thorin, the runes name Orcrist, the Goblin-cleaver in the ancient tongue of Gondolin; it was a famous blade. This, Gandalf, was Glamdring, Foe-hammer that the king of Gondolin once wore. Keep them well!
The Hobbit Chapter 3: A Short Rest
Page 48 (George Allen and Unwin 1966 paperback edition)
Note that Elrond not only translates the names of both swords, he also implies that Orc and Goblin are the same word.
In keeping with the pattern established in the preceding chapters, this chapter introduces yet another a new race: the goblins. Like the elves and dwarves, goblins already had a long history in Tolkien’s writings predating The Hobbit. Even if we overlook the undifferentiated fairy-folk lumped under the ‘goblin’ label in ‘Goblin Feet’ [published 1915], goblins were featured prominently throughout the early Silmarillion material, especially in ‘The Tale of Tinúviel’, ‘Turambar and the Foalókë’, and ‘The Fall of Gondolin’ [all written 1916–20]. Goblins fought alongside balrogs and dragons in the sack of Gondolin, and goblin-mercenaries aided the dwarves in looting Tinwelint’s caves in Artanor (the precursors in the legendarium to Thingol’s Thousand Caves of Menegroth in Doriath). The terms ‘goblin’ and ‘orc’ were used more or less interchangeably in the early material – thus in ‘The Fall of Gondolin’ we hear of ‘Melko’s goblins, the Orcs of the hills’ (BLT II.157), ‘the Orcs who are Melko’s goblins’ (BLT II.159), and ‘an innumerable host of the Orcs, the goblins of hatred’ (BLT II.176), while in ‘Turambar and the Foalókë’ Beleg tracks ‘the band of Orcs . . . a band of the goblins of Melko’ (BLT II.77). It’s possible to read Orc as the more specific term and goblin as the more generic, but often ‘goblin’ apparently replaces the more common ‘orc’ simply for the sake of variety, especially in the alliterative poetry. On the whole, the evidence suggests that Tolkien preferred ‘orc’ for works in the direct line of the Silmarillion tradition (such as ‘The Sketch of the Mythology’, the narrative poems that make up The Lays of Beleriand, the 1930 Quenta, and so forth) and used ‘goblin’ in more light-hearted contexts, such as The Father Christmas Letters and The Hobbit.
John Rateliff - The History of the Hobbit - The Second Phase - IV. Goblins i. Goblins
