I don’t think Cameron did much serious negotiation on this front. He resigned the evening of the referendum results.

It was a deliberate choice by his successors to pursue a hard Brexit. A softer Brexit was always possible and likely would have been more acceptable to the EU, as it would have had less financial impact and disruption to trade.

That's what happened afterwards.

The referendum was triggered by a failed negotiation between Cameron and the EU in the first place.