SW falls near the Spring Break peak season and thus, yep - resort prices are among the highest of the year.
SW has a more limited appeal than other race weekends due to its theme.
It's HOT down here in April. Not as hot as May through Oct., but almost always significantly hotter than Marathon and PHM weekends, and often hotter than W&D.
I think all of those issues factor into SW maybe not seeming as popular as other race weekends. But you also have to consider that races selling out at all, let alone quickly, is only a fairly recent phenomenon. In 2012, I registered for the PHM 5K only a month before the race.