This was a more expensive ride to build and maintain than a seasonal park could afford to support without the backing of AB anymore. It was a good ride and it will be missed, but ridership had really dropped off over the years and if it was a choice between maintaining Darkastle or some of the other things in the park (Loch Ness Monster, steam train, skyride, flume) which are older and can't be replaced, they made the right decision.
A company in a delicate financial situation has to sometimes make hard choices.