I think, from conversations I've had, and my own gut feelings, is that the 'stay and reform' thing died with Cameron getting the big finger when he sought to fiddle with the relationship - I think that regardless of you view of what Cameron was asking for, the EU telling one of its big members (snort, fnaar, gurgle) with 1/7th of all economic activity in the bloc, half it's P5 seats on the UNSC, half it's nukes and about 30% of its usable military force, that it could fuck off, said that serious change in any direction was simply not on the cards.
It very clearly signposted that 'the project' was the important thing, and absolutely not the servant of the member states. That turned a lot of people (probably those of soft remain, who saw the relationship in transactional terms) right off.