At the point the airline decides to scrub it for the next morning, it's almost a cancel/run the next day as opposed to a delay. ie, some of that time is just because they probably can't reasonably take off at 2 or 3 AM.
It was quite stormy last night, rained a lot of the night. Upside, it's clear now and looks like it should be reasonable today and tomorrow, so ORD should get caught up.