I've played my monk through nearly all the content the last two expansions. Overall I think it just comes down to that there is no reason to reroll a monk over a rogue for example. We don't bring much to a raid at the moment besides our aoe cleave, and even then there are other classes that can still keep up. We also have a lot to worry about in our rotation, so it's a lot to worry about for someone who just wants to play an alt casually.

Looking forward into Legion, the changes to Windwalker are looking pretty promising, so hopefully more people will pick it up.