I think it's something to do with bringing each of their parks up to the same level or something? Like, Alton, Chessington and now Legoland all have a hotel, with only one of the parks (Alton) actually having the need for a more than one day stay.
Chessington, for instance, is pretty much in the same boat as Thorpe, apart from Chessington is aimed at families with younger children, most of whom in my experience can't usually afford to stay in a hotel just for the sake of it. So there must be some kind of reasoning behind it.
I don't know what's going to happen with the Thorpe hotel. Like Furie said, it generally is frowned upon for under 18s to stay in hotels unaccompanied (I don't think it's an actual 'rule', I've been able to book hotels for myself before under the age of 18) and yet, that is Thorpe's target audience? Teenagers staying overnight with no parental control are going to be an absolute nightmare, so I wonder how they're going to handle it.