I have a few theories. None of its concrete evidence and is pure conjecture on my part.
- The failure of the Swarm. We all know that Merlin were initially disappointed by the Swarms first season. Many reasons were cited (which I'm sure those in the know can find) but essentially, I think this lack of interest in the new coaster dented confidence in what the park could do. Baring in mind, 2011 saw one of the biggest years the park had and that was on the back of Storm Surge. Cheaper investment bought massive returns.
- The next Big thing. I think Thorpe has fallen into the trap that Six Flags did. By appealing solely to the thrill sector you severely limit interest. Teenagers want the next big thing and in the 00s, Thorpe easily provided that thrill. Hit after hit, success after success. Coasters galore, several years where we got two flat rides. That investment was unsustainable. Now though, rides like Colossus, Inferno, Tidal Wave, Detonator & Vortex are old hat. They are unmaintained and look their age. Thorpe were unable to drum up interest with The Swarm, Derren Brown opened late and was pretty weak and the less said about the reliance on IP to drive interest the better. The park has a real image problem which leads me to..
- Branding. Thorpe has a huge perception problem. Is it a thrill park, is it a park for up for it families, do we make X a family coaster then two years later refurbish it for adults. Do we change our market to families and then close the biggest family ride we've got (Loggers Leap). Do we advertise our park in our 2019 advert or have an image of goats, donuts and a hotel room. None of this evokes excitement. It smacks of a park throwing anything, everything at the wall to see what sticks and new general managers are not given enough time to evolve/change the place before they are fired and a new one bought in.
- The Smiler crash. Ironically, four years later, I feel like the crash has had this weird echoing effect at Thorpe compared to the other two main Merlin parks. It's almost like it never recovered from the initial drop in attendance whilst Alton has recovered well thanks to Wicker Man and Chessington never saw the drop because of its family audience. You could say the lack of investment is really hurting Thorpe right now but then there seems to be no steps to change this.
- The park itself. It's fallen a long way since the hefty days of 2006. If they are to recover the place they need to sort out their offering. Having abandoned flat rides and log flumes standing doesn't help, neither does the game stalls or the lack of maintenance and paint jobs on their rollercoasters. The basics need to be sorted for Thorpe to recover. Chessington was exactly the same way in the 00's but that now feels like the strongest Merlin park IMO. If Thorpe can sort this then they might be able to get back to the top.