I'm actually suprised Wildstar has not enjoyed the same level of success... there was an awful lot of interest in it initially but things seem to have gone quiet with it of late
Wildstar hasn't done too well because of multiple reasons.
1. They catered to the minority crowd who screamed for Vanilla WoW and TBC days, with attunements and 40man raids. Then when they got that, they realised they didn't want it after all. The same happened with SWTOR, the minority screamed for no LFG and 3 months after launch, they add LFG due to community pressure. Generally, the player base have no idea what they want in a game.
2. The community cried for more servers and they opened more and paid the price.
3. They completely underestimated the importance of the casual MMO player. Appealing to a niche is great, but forgetting about a large portion of the MMO player base is stupid.
4. The community don't understand the MMO cycle and the general Psychology of a MMO player. The average player is coming from a game to try the new game, they compare the new game to their current game and realise its either better or vastly different that its off-putting. Wildstar for most people i feel was vastly different in that it was harder then most MMO's, the combat was different and the 20/40man raiding coupled with the attunement process and brutal dungeons.
5. They tried to take on WoW and from experience if you do this, you loose and they have lost.
There are other reasons, like bugs, but for the most part the above reasons are what i've found from speaking to a lot of the players i interact with.
If the above points where not there or less dominant, i would suspect the game would be doing vastly better.