I suspect it has a lot to do with the rise of relatively cheap global travel and strong disparity in the median wealth of different nations. 100+ years ago could you travel from a poor country to a wealthy country with relative ease and expect to raise your own standard of living? Either the journey was expensive and arduous, or the situation in places you could travel to was not likely to be a massive improvement on your own. There are very strong exceptions of course; Irish people leaving the famine come to mind, but it pretty much came to people dying to push them to make the trip.
Also, 100+ years ago an immigrant was less likely to be viewed as a burden, because there were fewer (if any) government supports for regular citizens. I'm pretty left politically, but I acknowledge that the more government spends on its people the less likely the government (and the voters) are to be inclined to let newcomers in who are not immediate contributors.
States in the US have different laws (consider income tax rates), and travel inside the US is super cheap, but we don't see people migrating like crazy to avoid taxes or get free stuff from the government.
I think "to avoid taxes or get free stuff from the government" was an essential condition on what you just quoted. And for what it's worth I think I agree -- I don't think I know anyone who has moved to a state mainly for tax reasons. (Though I do know people who have avoided moving to California, allegedly because of taxes.)
Though maybe the fact that people can move to take advantage of state giveaways, is a big constraint on the way they're currently designed? What conditions does Alaska put on its Permanent Fund payments?
People change states all the time, and generally for reasons you might think. It's not so much "I want to lower my tax burden" but "my paycheck will be higher and my housing costs lower - and the schools are better!", which of course is often related to tax policy.
I beg to differ with your last argument, France and USA have really different approach in how they support citizens, yet both are currently really opposed to let newcomers enter their borders.
I think one problem is like you said, the huge improvement in ones' situation when moving to a new country.
Why is there such a big difference ? Why money is concentrated in so little space and groups of people?
France and the US, while different, both offer far better supports to their people than most of the world enjoys. Social security, free public schooling (admittedly of dubious quality), physical security, etc.
Also, 100+ years ago an immigrant was less likely to be viewed as a burden, because there were fewer (if any) government supports for regular citizens. I'm pretty left politically, but I acknowledge that the more government spends on its people the less likely the government (and the voters) are to be inclined to let newcomers in who are not immediate contributors.