Stop providing camper van/ caravan places and replace the space with more car parking.
I speak as someone who has attended every Glastonbury since 2003 and half of them in my van but in my (unpopular) opinion, camping outside the fence is a lesser experience and encourages too many old biffers to keep coming. The festival should be something you fully participate in rather than attend/observe. Whenever I've taken my tent in, and I last camped inside the fence in 2019, it's been a more immersive experience and I've contributed more to the place.
Go into the camper van fields and it feels more like a caravan club rally than a festival with neatly pegged awnings, gas barbecues, satellite tv dishes out and moaning oldies going to bed at 10pm.