Three major reasons in combination. Their new leader performed particularly poorly in electioneering, and associated with this they nailed their election colou

Three major reasons in combination. Their new leader performed particularly poorly in electioneering, and associated with this they nailed their election colours to "Bollocks to Brexit", and repealing article 50 legislation. i.e. Under us, Brexit is not happening, regardless of referendum, calls for second referendum or rerun with more honest campaign rules or what have you. Last, I think (I'm guessing) there was a good degree of everyone being beyond sick of this, and vote Tory so we're out and it's over and done with. Of course leaving is just the start of a 10x longer period of trying to negotiate with everyone, and make things like import/export work again, preserve NI peace etc... Brexit will continue to be the story of the coming decade -- all of it. Our famously resilient and politically neutral civil service is already deeply damaged by it. Minor reasons are a lot of very amateur hour election leaflets with faked graphs, pretend newspapers that made them look like the Liberals in the 1970s once again -- intellectual, worthy, and utterly irrelevant. They also split the anti vote in more than one constituency.