Someone posted recently (can't remember where I saw it or I'd link to it to be sure I'm not misrepresenting their comments - so if someone can find it, I'd be obliged) that fantasy worlds should better represent real life. Not be modern or anything like that, but have more variety within races (there are quite a number of different shades and cultures of humans on earth, why aren't there that number of elves or dwarves, etc). It's a good question. One I've never asked before. Why aren't there several shades of elves? And are they necessary?
Races in fantasy sprang originally from fairy tales. Coming from European backgrounds, these races tended, like Europeans at the time, to be uniform. Tolkien 'updated' some of these races (making Elves human height for instance) but left other things the same, mainly their general uniformity (I did mention in my last post that he had more than one type of Elf - forest Elves that pop up in the Hobbit and are disliked by their more cultured cousins from the Lord of the Rings). But for the most part there is little distinction.
I imagine, like me, most authors stick with the classics. Why invent new monsters and races when mythology is chock full of copywriteless creatures? And they can be used in so many ways, tailored to fit each world's individual needs. Want your elves to be taller than humans? Shorter? Live 500 years? 5000? Only 50? Have pointed ears with lobes? Without lobes? 'Not much variation' you think? Well, consider that each creature, each world, each character and each plot has variations and the number of books you can get from them are endless. Some of these ideas are so standard as to be almost cliche - like the members in the questing band (the thief, the warrior, the magician,...). But you can get some pretty original ideas as well.
Even with worlds like Forgotten Realms, the traditional ideas were not tinkered with too much. There are surface (happy, good, white skinned) Elves and the Drow (dark skinned, evil Elves that live underground). Why did they only create 2 kinds and why are they using what must be (nowadays - remember this idea is over 15 years old) considered racist? Is it the simplistic 'white hats' vs 'black hats' mentality that created this? I don't know and for my purposes here it doesn't matter except to say that a lot of novelists have followed their lead and having 2 races of Elves is now an almost traditional element in it's own right. In terms of dealing with racism in the books they've used their races to good purpose. Most notably in the character of Drizzt Do'Urden, the drow warrior who forsakes his dark kin and lives on the surface. In each of the books he has to overcome racial prejudice. But I digress.
This doesn't explain why more modern fantasy doesn't include more varied subraces. One idea (and probably mine, as I've only got one race of Elves in the books I write) is that there's simply no time. There's no time to explain why you have so many subraces. There's only so much space in a book. If I have to explain the world, the history, the characters and tell the plot I don't have room to explain the entire evolutionary background or introduce dozens of smaller tribes - unless doing so adds something to the novel. In other words, keeping to a few standard races allows for differences without bogging a book down with too much explanation (now, there are authors who can accomplish all of this, and I take my hat off to them, but I understand why the majority don't bother).
Another possibility is that many books take place in a small geographical area. Given the traditional medieval setting (which not everyone uses but enough do for me to emply that in my example) travel is difficult and touring the world likely unrealistic. So encountering a few new races and varied cultures is understandable, encountering dozens, probably less so (unless you end up a central trading post where people from all over the world congregate or something).
What I'm saying here is the complaint is valid but there are equally valid reasons for why we're not seeing it addressed. Maybe people will address the point in the future. Maybe they won't. So long as people keep writing good fantasy I'll keep reading it, whether they use 1 race of Elves, 20, or none.