I think you're right that Tolkien's focus changed  from "myth-building".  Maybe he moved toward "world-building" -- a need to give ME a more "realistic" history complete with environmental disaster, native peoples who are displaced, plagues, details battle formations, and histories reconstructed from disparate sources.

Tolkien himself said that the desire for details was seductive.

"I am not now at all sure that the tendency to treat the whole thing as a kind of vast game is really good-certainly not for me who find that kind of thing only too fatally attractive.  "  --Intro to UT (from a letter written in 1955.)

It would certainly also be easy to forget things you had written before as well.  Especially when your earlier "version" was one or two explanatory sentences.

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Field guides explain color,
vagrant range,

the vague translations of a song,
but nothing of the distance
that develops into absence.
                -Walter Pavlich