Serious question: Why? We were a contender literally one year ago, with him on the sidelines. This year we had a young team with a bad fit at DC and got off to a bad start we could never quite pull ourselves out of. All of that is very fixable.
At this point I'm not opposed to making a change. I think Kelly may be burned out on the job. But I also think he can turn it around. Especially with a few changes to the coaching staff. We should have a high bar for replacing him.
The bad fit D coordinator was his own fault for not making a move earlier. It was fixable in the offseason and he looked the other way. IMO, he's done the same thing with a few members of the staff, namely Luongo and Booker. Like others have already mentioned, he's made questionable decisions lately. I'd like to know more about the process he uses to grade or evaluate his staff but he seems to take a really long time to make changes.
I don't believe his offensive system is built for college success, specifically the running game. There's just too much lateral movement in the run game and not enough getting downhill. That type of running leads to no yards or negative yards far too often and then we have to rely on chunk yardage plays to keep drives moving forward. I believe the mere system, not the execution, is why we see offensive inconsistency. I prefer a system that relies more on downhill running and positive plays to create third and manageable situations. It makes it much easier to have a consistent offense. I'm not suggesting we play "man ball". I'd be fine with winging it, provided we run straight forward when it's not getting aired out.
The schedule was manageable this year. If great coach or even a really good coach was on our sidelines, we could be undefeated. That has really frustrated me. Arguably, we've had more talent than anyone on the schedule this year yet we've come up a possession short more often than not. That's on kelly, whether he hasn't developed the talent, made poor decisions regarding his staff, hasn't installed the proper game plans, hasn't made correct in game decisions, hasn't motivated his team to "want it more" than their opponent or some combination of all that I've mentioned (and I think it's easy to argue he's done all of these).
I respect Kelly for what he did for the program and he's going to leave the cupboard relatively full for the next staff. I hope he lands on his feet. For the reasons stated above, I think it's time for a change.