easy, its just not as good as wow, thats basically the reason.
this starts with its accessibility, extremely long load times, it takes forever to change zones, and even after its loaded you still have stuttering when lots of players are around and on other occasians. endgame does not offer as much variety as wows, raids feel not half a polished as wows. now the question is, why would you play the second best MMO if you can play the best one? there are basically two reasons.
1. you just cant see wow anymore because you played it seven years.
2. you love star wars
both of them apply to me. thats why I still play it and I guess I will for a while. and I think that also applies to most of the <1 000 000 subscribers the game still has.