The reason is really simple:
1) A ton of people played Wotlk to 'defeat' Arthas.
2) A ton of people want to defeat Sargeras.
3) Wotlk has seen highest number of people playing WoW, due to #1.
4) Sargeras expansion could/would fair the same as Wotlk, due to #2.
5) Lots of people quit after wotlk due to ticking off #1.
6) There is a high chance even more people would leave after ticking off #2.