All of these countries have majority populations of white Europeans mostly descended from locals... which is to say that they each have a majority population that is of one ethnic and sociocultural background which they have shared for hundreds (or thousands) of years. Their internal social stability has a lot more to do with the historical momentum of their population having primarily shared interests and culture than it does with their method of voting.
1/4 of the swiss population are immigrants, not including naturalized immigrants. And most countries have a majority population...what exactly is the argument there?
It's mostly because this has been a go-to response by Americans in topics about some European countries doing significantly better than the US: "It's because they don't have black people".
Tbf that's more them telling on themselves in context.
Roosevelt's coalition failed to deliver the full extent of what we'd see in Nordic states and other such robust welfare systems because the dixiecrats rebelled against those things also applying to black people and not being easy enough to use discriminatingly.