People voted for leave on the basis there would be no downside. Just read the quotes from prominent Brexiteers posted in the other comments...

While some people were undoubtedly ok with a downside (especially if that cost was bourne by others, e.g. pensioners with guaranteed incomes and no jobs to lose), many leave voters accepted assurances "this would be the easiest negotiation ever". Remainers pointing out there was a downside were roundly condemned for "talking down Britain".