It's for the same reason that movies about the underdogs succeeding against all odds gross ridiculous amounts of money at the box office. The world loves a Cinderella story, seeing someone overcome everything and show that it's always possible (though not always probable) to triumph no matter what makes us feel that we could do it too, that there's always hope. Cheesy, I know, but I'm pretty sure that's what the pull is.
Me, I usually root for whoever doesn't come off as a huge jerk. But maybe that's it too, we see the opposition to the underdogs as overconfident snobs who need to be brought down a peg or two.