Differences in the economic profile of different areas has always spurred economic migration, be it across small areas or across oceans.
However, traditionally, going back even to the middle ages in places like the UK, people overall moved a lot less, and actually quite a lot of the labour force wasn't even allowed to move, which of course we would not accept today on the basis of individual movement rights within nations.
It was the rise of industrialization and changes in the early 20th century that created a much more mobile population, more like what we know today. But it's really important to understand that the mobile workforce has been something that, more than anyone, is beneficial for capitalists, the people who own big businesses. For much the same reason the elite tend to favour free worldwide movement of capital in the banking system, they also tend to push for laws and social structures that make for a mobile workforce - one that can be redeployed and retrained at will.
This strongly advantages the elite, but it is much less of a clear benefit for the workers. It tends to leave their communities more hollowed out, and also it means that workers from less economically successful regions will migrate into their communities. It can as a result of both these things leave workers and their communities more dependent on help from the state because their communities are less self-sustaining - and that is both in the communities with work and those without.
This is why, traditionally, the labour movement and the Labour Party was always very concerned with the movement of Labour. It was a major reason for EU skepticism in the LP historically. The move to being not only pro-EU, but actually denigrating those who aren't, and saying they aren't fit to vote Labour, is a major change, almost an about face. And it should not surprise anyone that for wc people it looks like they are being spit upon for maintaining the exact same concerns that relate to their own economic and community success that they always have.
People can't always articulate it but it has left a bitterness that it will be difficult for many people to forgive, and it affects how they see the LP on almost any issue.