Various reasons, e.g. losing the perceived economic benefits of a free trade zone with the rest of the EU countries.

EDIT:

I can't reply to the comment below for some reason, so here's what I was going to say:

Much of the EU's unpopularity amongst the Brexit-supporting section of the UK's population comes from it repeatedly being used as a convenient scapegoat for domestic political issues, and the country's most-read newspapers printing decades of nonsense ("EU bans yogurt and cheese from school dinners", "EU bans straight and bendy bananas", etc.) that were taken as fact about what a broken institution it is.

Brexit was really just the end result of a spectacularly successful sustained propaganda campaign.

If that's all the eu were I'm sure it would be smashingly popular.

Losing access to the schengen zone travel and trade wise is a big deal. IIRC EU market is largest single market, or was. 'The Troubles' was really devastating and people rightfully fear going back.