I believe it was because they weren't endangered. ZSL & Whipsnade suddenly became very orientated toward endangered species and it may have been during the last recession when they were trying to cut costs too. But it left 'Bison Hill' with no bison.
I think only Whipsnade and Howletts/Port Lympne exhibit both Bison species in the UK. Woburn used to but I am not sure about now.
