People migrate in massive numbers to the US from Mexico and Central America, too...and most of those countries are at least as capitalistic as the US. The reasons are copmplex...as they are in the migrations you cite. There's a whole lot of migration to the democratic socialist countries in Scandinavia, to the UK (quite controversially at the moment...), to France, etc., too. Those migrants come from much more capitalistic countries.

But I will reiterate: statistical analysis of massively multivariate scenarios, which any comparison of the relative effectiveness of varying mixes of capitalism/socialism in two countries would be, is a frightfully complex matter.