I'm guessing the biggest grounds with terraces that remain are around 15000 capacity for rugby (don't follow rugby league, but assume places like St Helens / Warrington can get fairly full) and probably somewhere like Wycombe / Mansfield for football?
With 10 - 15k people, it's a lot easier to do a safe evacuation from a ground than with 35k people.
Doesn't explain why the cut-off isn't capacity based, beyond the scope to game the system with lots of clubs having a 19987 capacity or similar though...