The UK already had exceptional status on various rules - they were more a half-member than a full member. So clearly, the EU already cared enough to make except

The UK already had exceptional status on various rules - they were more a half-member than a full member. So clearly, the EU already cared enough to make exceptions. The Brexit vote was different: the UK government chose[1] to interpret its results as a requirement to leave the EU. That's not something an outsider can upend without interfering in the day-to-day governance of the state. And no member state wants the EU to interfere with its sovereignty on that level. So the only possible outcome was acceptance. On the other hand, had Cameron gone to the EU with a 90% "less EU" mandate in hand, the EU would either have to accede or kick the UK out. Kicking out a member state... pfffft. I would guess it can be vetoed by any member state - or at least any other (not being kicked out) member state. And some would - their EU track record is such that they could be next. Basically, the EU had no choice whilst respecting sovereignty of Member States. A less/more vote would have put the onus of kicking out a member on the EU, something I don't see the member states reaching a unanimous decision on. Which would leave negotiation.