Okay, so Senator Clinton isn't going to drop out until there's a nominee. After all, Huckabee took this approach and didn't leave the race until Senator McCain became the presumptive Republican nominee. But that was March 4!
The Democrats and the Obama campaign need to make up for lost time, and as long as Senator Clinton hangs out and overextends her stay, she eats into Senator Obama's news cycles -- drawing media attention away from him. Bad for the nominee, good for Senator McCain. And if it's about allowing voters in the remaining several primaries to have an opportunity to choo-choo-choose her, she can still do that even if she steps aside.
Meanwhile, she's just being disruptive now and the only option is for the superdelegates to step up and potentially embarrass her.