it's not always unpopular, it's unpopular when the integration costs (as in both actual budget but also cultural/psychological) become bigger than what the population expected.

for example look at Hungary. immigration is very unpopular at the polls, yet now the same government that created this sentiment through endless propaganda passed laws to allow more immigration of laborers, because the new Chinese battery factories that are under construction will require a lot more unskilled labor than which is available. they try to limit any kind of anchoring, no family unification, no visa for non-working spouses, etc. but it's not clear that this will actually attract enough workers...