It's even a bit sillier than that. David Cameron promised a dumb thing to get elected, on the assumption that he'd be in another libdem coalition, and the libdems wouldn't allow it. Then he got a majority. And campaigned against his own dumb thing.

Not only was it expected to fail at referendum, but the referendum itself was never really meant to happen.

> What's shocking of course is that the Brexit vote didn't end up the same way ("ok the vote passed but it can't be done anyway").

It went very much the opposite; they went for almost the most extreme possible version of the thing. A lot of people voting for Brexit had been sold on, basically, Norway's status (effectively in the EU, but can pretend not to be, and has its own rules on fish and can have whatever weird passports it wants). This is part of why support for it evaporated so quickly (the last time any serious poll showed staying out as having more support for rejoining was early 2022); the delivered product was simply not what most people had been sold.