Alton towers has to account for the family. Alton towers must account for the children who the whole UG land theme is based on. Before Rita it had a lot of attractions for the family, so the Family needs to be entertained one way or another, otherwise the area would not appeal.
Alton towers has accounted for the family, Rita has it's own personality of UG land, and UG land has a nice family personality. It all matches in a ultimate feel. Who cares if it doesnt have matching theme, if it work it works. And that's why the GP love it.
Thorpe park is more flexible, teens can get there on there own by bus, and ride Stealth and the thrilling rides, and theming isn't essential. So the family aren't attracted to the area and go to another park like Chessington where theming is necessary. It's a obvious cheap short cut for Tussuad's.
I hope you catch what I mean.