The decision was made to have it on Sat and Sun only because many people, understandably, pack up and leave early(ish) on the Monday
Therefore, if it was held on an ordinary weekend (and how would we choose which weekend!!), it's likely that people would pack up and leave early on the Sunday.
Having camping available Fri, Sat and Sun nights means people can show their vehicles and socialise etc for 2 full days then leave whenever they want on the Monday
Hope that explains our thinking