I have no idea why officially, but, to me, Kratos fits much better. I know Sweet Tooth's origin story and everything, but most of the time we see him driving around in a car. Kratos, on the other hand, is beheading people, ripping foes in half, and being all-around brutal.

It also doesn't hurt that he's collected so many items to use over the various games so they could be as special moves.