This is because these mounts have unique special abilities. Water striders can walk on water, golems can gather materials without dismounting, and yaks and mammoths have vendors so you can sell and repair.
WoW should spread out these special abilities to other mounts. Ideally, every mount should have one, and only one, special ability. That way you would have a variety of mounts to use in different situations.
- Flying - can fly
- Fast - can move faster than normal
- Gathering - can gather without dismounting
- Vendors - has vendors
- Two-Seats - can carry a passenger
- Swimming - provides water-breathing and faster swim speed
- Steady - cannot be dazed or dismounted
Of course, this is a nerf to Sky Golems and some Two-Seats mounts as they effectively have two special abilities. Personally, I think the Sky Golem is a little too good, and could stand a nerf.
Another possibility is to come up with a few more properties, and then let each mount have two special properties. That would lead to a bunch of combinations.
I think making sure all mounts have access to special abilities would make it more likely that a wide variety of mounts are used, and reduce the importance of getting specific chase mounts. Personally, I'm a little tired of seeing everyone on water striders, and would like to use a different mount every so often.