These mounts are supposed to be RARE, they're supposed to take TIME and DEDICATION to get.
Slap a raid of 20 people together and blow through every rarespawn that spawns, with each person having their own chance of getting a mount? In three hours, everyone in the raid has one.
Make the droprate incredibly low? You just have people dropping out after they get one mount, or people ending up with multiples of the same mount, while other people continue to jump into groups and get none.
And what if you can solo it? What then? I don't need a group of people to kill the mob; I think my chances should be higher than someone who needs three, or five, people to kill it. But you can't put in some sort of "scaling system," where the drop chance decreases with the more people you have attacking it, as it just goes back to people interfering with one another again.
I'm perfectly fine with the mobs (again, baring the resetting) the way they are now. I began by soloing them, until I joined up with two other people. We farmed HARD for these mounts, until all three of us came away with one.
If you still want a direhorn mount, you can wait until you're exalted with the new horde/alliance forces, and then buy that color. (That's more than we're getting for the new kick ass skyscreamer mount; you have to be lucky enough to be in a raiding guild to get a shot at that within the moderate future, and they didn't even choose a good skin) What is required to obtain one of THESE specific mount colors has been laid out, and either you're game for it, or you aren't.