The short answer regarding JRRT's work:
I created the D&D game to sell, to get to as many consumers as possible, and the best way to do that was to include races that were favored by the many young Tolkien fans. Dwarves are hardly a main feature of the Rings trilogy, but hobbits, elves, orcs, balrog, and ents (the Anglo-Saxon word for "giant," of course) were. So I included them so as to sell the game. Even though I find the Rings trilogy a bore, found
Bored of the Rings great sport, so what? Gamers are able to enjoy their D&D campaign with as much Tolkein flavor as they wish.
A careful analysis of all the races and creatures in the OAD&D game will see how small the percentage of Tolkien-inspired material there is actually within the roster. Of course, I do not count elves as JRRT's creation at all, they being the stuff of much British folklore, as Dwarves are Scandanivian and Germanic. Of all that the Professor wrote about, the hobbits, the balrog, and the ents were all that were uniquely detailed.
Speaking of bored, that's what this tedious subject does to me and quite a few others I should imagine
Cheers,
Gary