Why is J.J. Abrams not directing Mission: Impossible IV?

Like I know hes producing it alongside with Tom Cruise and hes also part of the screenwriting process, but why is he not sitting on the directors chair this time? What is the meaning of this? I mean, okay I know i'm over exaggerating now, my bad. Other than being the brainchild behind Lost, he has done an amazing job with the 3rd M:I movie, and with Star Trek, and with Super 8 (which I saw two days ago, definitely one of the best films I have seen this year). I mean, don't get me wrong, I like Brad Bird, he directed Iron Giant which will always be my favorite (one of my childhood movies) and other sick kids films. Look, all i'm worried about is if Bird's animated-kids film style will “baby-up“ the M:I franchise. So now i'm asking two questions: 1. Do you think Brad Bird will do that in the end? And 2. Why did J.J. Abrams sit this one out? (By that, I mean not directing M:I IV).