I'm pretty sure Horizon was considered for the minecraft theme. There was 2 surveys sent out, 1 for towers and 1 for chessington. I'm guessing the feedback steered them towards chessington as the better for fit for the land. It also showed a minecraft themed waterpark and accommodation, both of which chessington are planning on building. I'm guessing for Alton this would have involved re-theming what they already have.
This doesn't mean to say that Horizon is dead, just that the minecraft theme worked better at chessington. My guess is that they are looking for an alternative IP for Horizon and possibly Walliams world / the dungeons area. There was also more of a priority for Towers to fix what they already have rather than spend lots of capital on a new ride. I would imagine that means Horizon will be somewhere in the future when towers feel that the more immediate fixes are complete. I'm also quite glad they have gone in this direction as Horizon would not fix the underlying problems at Towers .