Imagine you wanted the Baron mount, but instead of it being its own drop, you'd have to get Ashes of Alar, Anzu and Invincible. But you didn't care at all for any of these three mounts. It seems unnecessary to me to make someone get these three mounts, that that person doesn't even want, just so that he can get the one mount that he does want. I'm sure that a hardcore collector would maybe go after these mounts anyway, but some of us maybe only want the emerald drake. That doesn't mean that the emerald drake should be sold by a vendor for 10g. But it could just be its own drop, and I don't see how that is an unreasonable proposition and why I'm getting so much flak for that.
The whole idea behind this achievement was to get other drakes in order to get the "last one". Yet most of the drakes don't make much sense, as they are not part of the original five colors. They could've made an achievement where you would have to get the black, bronze, red and blue one. That already requires some work and even involves a low drop mount.