really the most logical explanation was that the Big 10 and Pac 12 thought the other conferences would all follow suit and cancel. It's pretty obvious that information was leaking to friendly media outlets (Teddy Greenstein, Pat Forde) about the Big 10 was cancelling, even before the Big 10 voted. My guess is that they thought everyone else would capitulate. Once the SEC, ACC and Big 12 said "not so fast" they have been forced into a position where they have to actually explain why the decision was made.
Just coincidence that during the DNC convention that they had commercials running showing empty Big 10 stadiums. If they rest of the leagues had thrown in the towel, the shots would have been of stadiums around the country and not just Big 10 schools.
Note that the Big 10 may end up proving to be right. But there really wasn't any reason to cancel when they did. It shows they weren't really committed to playing.