Mmm, I agree with MrW; most bands don't do festivals if they don't have an album to push.
Also, I think it was Gary from Snow Patrol who once said that it really isn't that much fun trekking from festival to festival, as it is a bit of a logistical nightmare, so there's that to consider. Why put up with all the hassle if you don't have to?
Then again, U2 are doing Glasto and they have never done a festival before...