Rebel1one1 said:

If memory serves Abrams claimed he was "too much of a fan" to take on the film when it was initially offered. Not to mention the fact that he pointed out his "loyalty" to Star Trek. Than shortly thereafter he is officially named TFA director. It's obvious that it was about financial compensation. JJ Abrams apparent loyalty to his iteration of Star Trek clearly was not going to sway him from the potentially highest payday of his career. Even if it sucks, it's a win/win for him because the money will keep him secure and he would be welcomed back to Star Trek regardless of success or failure with The Force Awakens.

Maybe the person convinced him they were interested in making a good film while making a shit ton of money. That in conjunction with him being a fan of the series was probably the deciding factor. People now a days act as if you can only have one reason for doing anything. As if there aren't any other factors in play when it comes to making decisions.

Also if this film sucks his career will take a huge hit. Sure he'll make a lot of money but his credibility is tarnished.