Might be to do with the pricing, its been very good for these sort of events encouraging more people to stay over. For example, the 18th December, a Santa sleep over for 2 adults and 1 child is £165, that includes breakfast, dinner, visit santa and entry to the theme park and waterpark and car parking!!
In comparison, I've just been pricing up a date to stay over, June 20th for one night for 4 adults:
Splash £230 (per person £57.50)
ATH £319 (quite a jump!) (per person £79.75)
ATH Moon Voyager £369 (per person £92.25)
Enchanted Lodge £265 (per person £66.25)
Enchanted tree house £825!!!!!!!! (per person bearing in mind it sleeps 8 £103.12)
I know the tree house sleeps up to 8 (though the booking process only lets you book for 4 people so it could be more expensive than this), but £825!! Even if you reduce this to 2 people, its still the same price. Bearing in mind you could book the Raddle cottage nearby which sleeps 10 and has a hot tub for £700, between 10 (i.e. £70 a night).
But.... if you do go for a treehouse, you do get extra superdooper special VIP parking