Because fairweather fandom is douchey as fukk. You get no real satisfaction when your "team" wins anything because you don't actually care about the team or struggled through any of the losses. When you just jump to the favorites it's like

we were supposed to win anyway.
I couldn't be any other type of sports fan other than the one who sticks by his team for that reason. When we do win, you feel like you really deserve it as a fan and it feels so much better. Picking someone different every other year brings no satisfaction and doesn't allow you to be really emotionally invested, and it's just way more fun that way.