Because the politician in the EU clearly have no care for the will of the people as long as they get paid. There is not accountability to the people they pretend to represent, regardless of nationality. A clear cut case, I'd have thought, of destroying any desire to remain.
But as I said, we need to ensure the more accountable national politicians don't treat the public with the same disdain; but there is a better chance of that since they will be nearer to the people, i.e. more local, than some far away European politician, who even got there from the awful PR route.