It's a combination of factors and that's likely one of them. One other big one, I think, is that there's no consistency in the race themes so there's no sentimental value around this race weekend and on top of that, I don't think the actual race themes for the 10 miler have been all that good, with the exception of the first one (Tower of Terror, which DID have some sentimental value for some people).
Sounds like this weekend is less easy to plan a trip around for some reason, putting aside the warmer weather. Marathon Weekend sells itself, Princess Weekend has a dedicated following and you generally know what you're getting theme-wise every year, and Wine and Dine has Wine and Dine.
The weekend itself is fun though. 10 Miler is a very nice course that hits Epcot and HS (without the extra road miles of the W&D half course).