Having saying that it would also be nice to have a KFC, Maccy D's or Burger King. They have KFC at Thorpe, so I'm not too sure why it's not the same for Alton?
Tussauds added branded outlets (McDonalds, KFC and Pizza Hut) to Alton Towers in around 1998. They seemed to be on a 10 year lease as in 2009 Merlin started converting them to being owned and operated locations instead of being leased. Pizza/Pasta buffet opened at Alton and Chessington and Fried Chicken Co at Chessington (FCC opened at Alton in 2010).
Merlin obviously didn't want to do Burgers at the time, so gave a short lease to Burger King (McDs probably wanted 10 years again) until Burger Kitchen opened in 2013.
But Tussauds didn't buy Thorpe Park until 1998, so everything is on different contracts compared to Alton and Chessington. Thorpe never had McDonalds its always been Burger King and KFC. Last year the Pizza Hut buffet closed and re-opened as Pizza/Pasta. I would expect Merlin to switch out BK and KFC when they can, but no-one seems to know how long they are leased for.
Now Merlin have made the Burger Kitchen, Pizza/Pasta and Fried Chicken Co brands accross Chessington, Alton, Thorpe, Legoland, Blackpool Tower and Warwick Castle, I don't expect them to ever bring back leased food and beverage options.
I had the Fried Chicken at Chessington a few years ago and it was fine, same as a burger at Burger Kitchen at Towers was fine. I know they are overpriced, but it was OK. We only go up for Fireworks now and usually have Pizza for lunch followed by a pasty in Mutiny Bay in the evening as this seems to be slightly more cost-effective.