-It likely gets easier to roll out due to moving away from the Classic code
-T11 is still very popular among raiders
-Cata heroics are (in)famous (c'mon, everybody kept saying how players got "better" after Classic came out, maybe let's test that in a more challenging enviroment)
-It's not going last more than a year anyway because there isn't enough content to stretch it that long (meaning you can also quickly focus on other things)
I enjoy Classic and i enjoyed Cata (except the farming DS for a year part), altough for different reasons.
There's also some people within the Classic audience that aren't going to touch Wotlk, because many see it as the breaking point where WoW departed its RPG roots, it's still going to come out.
Not saying it will happen, but i'm not going to outrule it, because they gotta do something after Wotlk if they want to retain "Classic subs", endlessly releasing Classic with some changes isn't going to cut it.