For what possible reason would they make them 2000g to purchase? as if the 5000 to get the skill and all the time doing instances over and over wasnt enough. They should be free after all that.
We try and add as many mount options as possible, with various requirements so that you see less of the same types of mounts, making the one you're riding feel that much more unique. If you don't want to spend the gold and/or run the Cenarian Expedition dungeons, you can purchase a regular vendor mount.