I'm going to go with this. It's not so much that it ended, it just expanded to the point where it's not really about just one mascot anymore. The consoles no longer have a "face" because of the implicit limitations of that. While Nintendo sticks with Mario for his Mickey Mouse-esque quality, Microsoft and Sony would be loath to pin down their machines (often promoted as living room media centers) to a single image. As much as Kratos sells, it'd be a pretty terrible idea to make him your mascot. You'd scare away more people than you'd attract. Same story if they go with Sackboy.