If I had to give three reasons, these would be them:
1. Nonstandard abilities mean taking more time to come up with interesting uses for things, as opposed to the more standard kit other classes get. My guild's main tank has been tanking since vanilla and plays every tank spec but BM, because she finds the mechanics to be too weird.
2. The weirder, more playful lore and aesthetic. There are people out there who cant stand levity or anything less serious.
3. The fact that they're the only class introduced post vanilla that starts at level 1. Yes, I know about the catch-up mechanics, but there are people who don't , believe it or not.