The primary reason is because all the Nascar fans are there for the Daytona 500, and do WDW in the same trip, the offsite hotels all market packages for this. There are many who actually could not get tickets to the big race who just go down as a sort of pilgrimage; then once they have done all the other Race Week stuff in Daytona, they head to WDW and just watch the race at ESPN.
For others, it's a long weekend at the right time of year, and one of the few for which the airlines don't go totally insane with ticket price hikes. (I'd say most Nascar fans drive to Florida, it's not a huge weekend for flying.)