because inovation is too risky. Gamers always say they want something new and innovative but the truth is quite the opposite.
Whats the best selling FPS franchise? Call of Duty. Meanwhile innovative FPS like Killer 7, Stalker, Metro, Deus Ex and others at best do ok.
RTS? Starcraft is the most succesful while others that are far more innovative lose out.
TIme and time again players say they want something innovative but always buy the same old.
When it comes to MMOs you are talking about a HUGE investment so what do you do? You make it close to the industry leader while adding your own unique things.
Games that try something different like The Secret World, or SWG end up failing hard because the truth is players don't want that. Players want what they are used to.
It is important to keep in mind that it's just a very vocal minority which keeps proclaiming they want something new, the rest simply can't be bothered to join in on those discussions prior to release.