There is a few core reasons to why I think Thorpe has reached its current state.
Heavy expansion, and then, nothing at all. Thorpe had a rapid expansion/investment rate up until about 2012. They received the cream of the crop when it came to investments. Merlin at some point seemed to be more keen in investing in Thorpe over Alton even. Ever since 2013 the park has seen to take a backseat, and their last proper investment was the nail in the coffin (DBGT). The Ghost Train basically ruined all hope of Thorpe ever regaining its status as a worthy rival to Alton Towers for the best park in the UK. It also ruined all hope of any good investment anytime soon. Merlin have no reason to pump money into Thorpe anymore.
Lack of a resort. Thorpe Park is barely a resort. It barely has a hotel, and honestly it offers the least out of the 4 UK Merlin Parks when it comes to non theme park stuff. Look at Alton, they offer 2 hotels, Enchanted village, the roller coaster restaurant that is open past park hours, a water park, golf, a ropes course, and entertainment in their hotels. Thorpe Shark is absolutely pathetic in comparison. Chessington and Legoland also have much better accommodation. I bet those hotels are cash cows too, why invest in a park with one small budget hotel when you can invest in a park that is a full resort (Alton Towers)
No image. Thorpe Park is like a middle age person going through a identity crisis. It doesn't know who it is, who its target audience is, and it shows.