The Towers not reopening is probably rooted in the stripped operating budget the park gets (like everything wrong at the park).
The park relies a lot on Scarefest to get a big boost at the end of the year and make a lot of money off the mazes. They have most their mazes now running right through the public parts of the Towers, so to reopen would need all mazes to be dismantled and rebuilt at the end of the year.
Right now they only need to rebuild one or two mazes elsewhere on park, so are saving two big jobs and a whole lot of labour, materials, time etc.
It's terrible that nobody can go in the Towers, but if they had the money to do a proper setup every year they probably would. You got to remember the entirety of the vast Winter Wonderland appears and disappears in a matter of weeks (incredible) but the mazes at Alton Towers take months. It's money.
The redeeming thing is that having the mazes in the Towers means people can enjoy the ruins location in a very unique way. Some might say it's sacrilege but as a huge admirer of the building and its history, I think it's quite fun really and (currently) doesn't do any damage to the building.