Do you know the type of FURY that would erupt from D&D players if WotC decided to exclude a lot of monsters that were in earlier editions?
There was mass anger because they chose to put FROST GIANTS in MM2 instead of the first one.
Middle Earth gets away with it because they didn't HAVE four dozen different monstrous humanoids gathered from 3 previous editions of D&D, in addition to all the other monsters acquired over the editions people want.
Oh, I agree that the reasons for Breadth make sense. It's just worth noting that there really is a trade-off here when it comes to names. You can put out creatures with exotic names and get players to memorize them for only so many creatures. The single time that an Arrow Demon appears, if it worth noting that it's real name is (as a random example) Callicantzaros but it is also known to the people of the shining seas as Yog-Nephry and to the plains men as . . .
It's worth reading an AD&D monster manual to see the critters that no longer make the grade in modern editions and how what looks (to modern eyes) as a limited set of monsters could be so impressive at the time.
So I understand (and even agree) with the WotC strategy. My thinking is that if you want cool and interesting names, reducing the number of possible opponents will really help to make each one a deeper and more interesting creature.
That being said, there is a reason why vampires and werewolves are popular (even Tolkien used them as forms for Sauron to shift into) -- people intuitively know what they are and don't need it explained in painful detail.