Bournemouth Gardens is the only car park in the area with motorcycle parking and EV chargers, so at least half of the car park needs to be open to retain access to those and meet demand for both event and non-event visitors. With any public area closure, there needs to be traffic management (cones, barriers, signage etc) to prevent entry into the "hired" area which comes at a cost...

The space left to play with would be slightly bigger than the current patch by a few metres, cost a lot more and won't really benefit anyone.