In a system of PR you can expect parties to be more representative of their voters. And the EU is enough of an issue for it to be a distinctive part of a party's make up. Parties are either one thing or the other, not tearing themselves apart over the EU as the Conservative Party still is. Because as you pointed out; Cameron was campaigning against Brexit, but a big chunk of his party wasn't. I dare say he wasn't leading the Tories so much.