Same reason why people are fans of anything else. I'm not going to be a bandwagoner. If you ask me, most TRUE fans are fans of shitty teams. For the really good teams (Miami) I'd say only 5% of their fans are real the rest are just cheering for a good team so they can win and be happy.
I live in New Brunswick where lots of people are Boston fans, I wouldn't mind being one of them with the recent success Boston has had in variable sports, but heck Toronto's Canadian and Boston is so damn greasy :P