This is where the politicians have fallen down. No amount of rhetoric about "economic growth" or "migrants benefit the economy" isn't going to make a GP appointment suddenly become available before August, which is the kind of "on my doorstep" negative consequence most people have seen. Worse, most of the "positive consequence" is sitting in some offshore bank account, not making everyday standards of living higher.
For decades, the economy has been benefitting from migration, even before the free movement principle brought in the surge from Eastern Europe, but also for decades fewer new houses, fewer schools, fewer infrastructure projects, fewer hospital beds, GPs, etc than needed have been provided. For much of that time, even raising this concern was not permitted, and any who tried were branded "racist" in order to shut down any kind of debate.
This meant that the UK was like a pressure cooker, and it blew some while ago and brought us such things as the Rotherham scandal, the school "Trojan horse" scandal, along with "Muslim Patrol" followed by "Christian patrol". Once discussion was allowed, there was a need to address decades of built up problems, but progress was always slow, and we were often told by the politicians that "we would love to, but the EU won't let us".
The less affluent areas suffered a disproportionate amount of the negative consequences of all this, whereas the most affluent areas saw most of the benefits. However, it's one vote per person, not one vote per £1000 in the bank, so the remain camp had a big fight on their hands, and despite this, Junker wouldn't keep his trap shut just for one more day, and had to tell the UK right before the poll that Cameron would NEVER get any further reform, so any carrots he dangled in front of the electorate based on further discussion and reform if we were to remain were suddenly declared void.