It could be that part of the 3 season runs contract is that those playing the Doctor don't want to be typecast. They want to move on and act in other shows/movies. I'm not 100% sure - it's also possible the BBC doesn't want the same person playing the Doctor for too long. I do know, however, that Eccleston, Tennant, and Smith wanted to play other roles. I think Capaldi would be happy playing the Doctor for longer. It's possible he'll try to go the Tom Baker route!