Britain's been decaying since at least 1940. Basically everyone left alive here knows only failure. The idea of any success is as alien and revolutionary as suggesting we all convert to a new religion or give up private property. So any time things look like they might be succeeding, people, from voters to PMs self sabotage.
The press helps by giving people plausible deniability. Letting them pretend they didn't know. Politicians help by enacting terrible policies they know will fail just to stay in power for 6 more months. And other groups help by blaming each other (the hard left blames the centre left, right wing nationalists blame right wing free traders).
That's why people looked at the expert advice (that we would be prosperous and successful and might end up part of a world super power of we kept this EU business up) and immediately quit. That's why people were so willing to accept the bullshit and pretend it was real (like that we could leave the EU and stay in the EU).
Brexit is but a symptom of this, a big one but just one. It infects every aspect of our national identity. From the housing market to jobs to education.
Until things get a lot worse people won't be ready to try succeeding. So here we are, 5 years into a(nother) lost generation.