IMO that is based on marketing. Neither is a very good expansion, not bad, but nothing to hollar about. But, with cata you got this WORLD CHANGING event - the big bad dragons and the nastiest dragon of them all. People didn't know catas awfulness until they bought and played it. With MoP - you get the announced features that were probably a bit of a turnoff for some like pet battles and such, also there was all this unwarrented panda-hate calling the expansion childish. I think you got more people willing to try cata and then became disappointed versus MoP I think you had more that weren't even buying it to give it a shot. Ashame really, I came back after 2.5 years away to try MoP and I'm still playing 3 months later enjoying myself still.
P.S. as an alliance player it was worth re-subbing to MoP just to get to purge Dalaran.