Why PPV?

okay I'd like to get away from all the excessive Cotto-Margarito posts, although it's still an interesting topic and branch away to start another topic. so here's the question. I've always wanted to know why boxing is one of the few sports to almost exclusively use PPV in broadcasting of it's fights. (granted the PPV's are once a month and for argument's sake you can make a case that HBO and Showtime do show a good portion of the fights on there channel, but that's premium cable, a whole nother story) so why does boxing do it?