Tolkien explicou por que ele mencionou Gondolin em O Hobbit?

O mais perto que eu posso dizer, não; Tolkien nunca abordou por que ele escolheu Gondolin como locais de origem de Glamdring e Orcrist; a referência remonta até o primeiro esboço, de acordo com There and Back Again: J.R.R. Tolkien e as origens do Hobbit (ênfase minha):

In the original draft of The Hobbit, Tolkien had not fully decided that this Elrond was in fact the same as the Elrond of the mythology. He is described as an elf-friend, 'kind as Christmas,' an attractive alliterating phrase that was later changed to 'kind as summer' to remove the overt Christian reference. And the passage where Elrond identifies the swords is considerably shorter; he knows them to be old swords, made in Gondolin for the Goblin Wars, and he wonders whether they came to the trolls from a dragon's hoard, perhaps from one of the dragons that plundered the city at its fall.

^{There and Back Again: J.R.R. Tolkien and the Origins of The Hobbit Chapter 10: "Goblin Wars"}

Na Carta 257, Tolkien menciona a história dos Dias Antigos como fornecendo "profundidade histórica" para O Hobbit :

It had no necessary connexion with the 'mythology', but naturally became attracted towards this dominant construction in my mind, causing the tale to become larger and more heroic as it proceeded. Even so it could really stand quite apart, except for the references (unnecessary, though they give an impression of historical depth) to the Fall of Gondolin, Puffin 57 (hardback 63); the branches of the Elfkin, P. 161 (hardback 173 or 178), and the quarrel of King Thingol, Lúthien's father, with the Dwarves, P. 162.

^{The Letters of J.R.R. Tolkien 257: To Christopher Bretherton. July 1964}

Isto é semelhante a um comentário que ele fez em uma carta anterior, falando sobre algo semelhante em O Senhor dos Anéis :

Part of the attraction of The L[ord of the] R[ings] is, I think, due to the glimpses of a large history in the background: an attraction like that of viewing far off an unvisited island, or seeing the towers of a distant city gleaming in a sunlit mist.

^{The Letters of J.R.R. Tolkien 247: To Colonel Worskett. September 1963}

Se a intenção das alusões aos Dias Antigos é essa "profundidade histórica", então Gondolin é a escolha óbvia; é facilmente a mais significativa das ruínas das cidades élficas e ocupa um lugar especial na estrutura narrativa do universo.

Da Carta de Tolkien # 15 Para Allen & Unwin:

The magic and mythology and assumed 'history' and most of the names (e.g. the epic of the Fall of Gondolin) are, alas!, drawn from unpublished inventions, known only to my family, Miss Griffiths and Mr Lewis. I believe they give the narrative an air of 'reality' and have a northern atmosphere.

E, como Jason Baker citou, Tolkien incluiu Gondolin para "dar uma impressão de profundidade histórica".