Because originally they were promised the moon on a stick and also the leave vote was a general protest vote by certain sections in poorer areas of the country. Now they are angry because they feel that their vote was not respected and that democracy has been/will be undermined if we don't leave. At least that's what I believe.
This is it in a nutshell.
The most bizarre thing is that the narrative around leave never asked people to think about what they wanted instead of EU membership.
What kind of position should we have in the world, what kind of relationship with EU and other countries, what kind of economic model?
Because obviously you can vote leave because you don’t like something about the EU (see Federalism, the army point mentioned above).
But once out, then what? Who are our allies and trading partners for starters? There’s this bizarre assumption that we can act as a stand alone country. That’s a profound lack of understanding around how the world works.
So while the ‘i don’t want a European army’ sounds plausible on the surface, it has to be considered in the light of alternatives. I don’t want to be Russia/China/USA’s bitch either.
Same with trade. If we have issues with their trade laws, what alternatives have leavers looked into? The much vaunted WTO has lots of its own laws that need to be adhered to.
Basically leaving the EU doesn’t happen in a vacuum. I don’t think many leavers truly get this.