I agree re: Greyhawk. It did indeed immediately start to move away from monocultures, at least with certain races.
Cool.
Part of the issue is the constant return to monocultures every edition, most spectacularly to my eye with 3E.
My take is that it's not really monoculture, since the settings were not profoundly modified. My take on it is that because 3e did a really nice job of rationalisation of basically everything, they left only one racial template per race in the PH, but knowing that the culture would come from the setting.
On the one hand, I can understand how you could see it as being back to monoculture, but I don't think it's sustained at all from the rest of the publications.
2E had the most branching out from monocultures (imho), and it had from the very start (c.f. Taladas), so it was particularly shocking to see 3E "reel back in" the races (and classes, but that's another discussion), and then it never really reached the same levels of varied cultures for races, and whilst 3.XE added tons and tons of races, virtually all were presented as fairly narrow monocultures. Even ones that it made no sense for.
My take is that it's because 3e did a lot of crunchy splat books which were supposed to be generic so that they could be used across settings. But if you crossed this genericity with the settings which did not suddenly become monocultural, I don't think it's a problem, or rather I think it comes from the combination of the ever-growing generic ruleset of 3e that still had to be combined with even more setting books, so you needed a lot of stuff to see past the monoculture. But it was not, at least for me, the intent.
Re: Tolkien-derivative races, indeed, but again, we saw people take them pretty far from that pretty early on, so it was curious that they kept reverting (except Halflings/Gnomes, who were in flux through 3E-5E to a greater or lesser extent - still monocultural or the like for the most part, but what that culture was varied a bit more).
For that, I think you might need to blame popular culture and the LotR movies, actually, rather than the game itself. Because D&D is generic, the PH has to avoid cultures, otherwise it might create conflicts with the forthcoming/continuing settings. But culture cannot be left out to describe a race, especially when you want to relate to the popular culture tropes so that beginners can make the link to the cool things that they might want to play.
So, don't you think that this apparent reversal to monoculture in each new edition is not anything more than a return to genericity at the start of a new, and therefore simpler edition ?