I think it is a combination of all the things mentioned by posters upthread. Superbowl, Daytona, Mardi Gras and other school vacations, cheerleading and other conventions, cheap airfare, winter weather up north, etc etc.
I tried for President's Weekend dates over a month ago, and the places I wanted were all booked up. (WDW deluxes)
If my hubby was going to the Superbowl, you can bet the rest of the family would be staying at WDW! (well, actually, I would want to go to the game too, but some/most wives would not)