> Majority of the UK population? Growing support?
Maybe not now, but at the time, definitely so. Don't forget they polled more votes than the SNP, who have many more seats in Parliament.
The only reason they imploded is because they were set up to fight the battle, not clear up the mess afterwards. Neither UKIP, nor Cameron, nor any sensible person, expected the vote to go in favour of brexit. Cameron thought, along with apparently politically-informed people, that offering the brexit referendum was an empty promise to silence the whingers. Not his fault he was wrong about that, though you seem convinced that blaming him as an easy target (because he's doing something else now?) is better than blaming those more directly responsible. There's a long enough list still active in UK politics, after all.