[headmelted]

You'll get a hundred different hot takes on this as it's a very touchy subject but in essence:

Conservative backbenchers have wanted out of the EU for a while. To keep the peace within his own party, David Cameron (the former PM) promised them a referendum figuring it was a straight-forward (and safe!) lay-up that put the matter to bed, and that would then be the end of it. Essentially, "Remain" would easily win by a landslide and the matter would be settled for good.

The plan didn't pan out.

Because it was essentially intended as a rhetorical question when it was devised, no great amount of time was spent figuring out what a Leave vote would mean in practice as it was not expected to happen. Unfortunately, the upper echelons of both political parties spent so little time and effort explaining the implications that it left that role wide open for media outlets to fill in the many blanks - meaning that people on the Left were told it would be the end of days and those on the Right that it was a free ticket to the promised land with no downsides. The Right vote carried.