People did not think it was in danger of passing so they didn't go vote. Others pretty immediately said they had voted exit as a protest and because, again, they didn't think it would pass and it was non-binding.

Polling made it pretty quickly clear the vote was a fluke. But they ran with it like a mandate from a super majority. Even as it became even less popular through the years it took to negotiate Brexit, when they could have cancelled it at any time.