Thing is, you go to a Disney park to experience the "Disney Experience", which includes the Magic Kingdom ride staples like Space Mountain, Big Thunder, Haunted Mansion, Pirates, etc. It is expected of Disney parks to have these rides/attractions, and even then they're not always exact clones of each other (Paris of course, getting different storylines/layouts which are arguably superior to the originals)...
You don't go to a Merlin park for the "Merlin Experience", because such a thing doesn't exist, as all of the Merlin parks (bar Legoland of course, which follows the "Disney Experience" method) are unique...
Gardaland and Heide are pretty much the lame ducks of the Merlin portfolio anyway... I'm not entirely sure why they still own them in a way because Garda especially is one of the worst performers in the park side of things...