I just copy and paste from other forum
imaging-resource just published the interview with the guy
Mike Owen/Canon: It's a variety of reasons, really. I mean, yes, there is the cost side of things, but what we've tried to do with the autofocus system on the 6D is to actually improve low-light performance. It goes down to EV -3. So when emphasizing low-light performance, we've always had to make elements of the AF system larger, which limits our ability to put in more cross-type points.
DE: So it's a conscious design decision. You really wanted to emphasize low light.
MO: Yes, absolutely. It was a decision that we felt that this type of camera in this particular user group, auto focus performance is not 100% critical. But low-light performance for autofocus is important. And it's just about that trade off.