Simple answer - global capital wants cheap labour and an army of rootless consumers. Pesky little things like democracy and nation states get in the way of this. Homogenous societies demonstrate much greater social solidarity than "diverse" ones. Countries with high levels of social solidarity are more likely to support higher wages, a welfare state and such things as minimum wage legislation and regulations that get in the way of a race to the bottom.
The quickest way for capital and the transnational bodies that support it eg the EU ans UN to gain control of societies is to undermine nation states by flooding them with culturally incompatible migrants. Solidarity is destroyed. People no longer wish to pay taxes because they know that they, and people like them, are unlikely to benefit.
Most governments are in essentially in thrall to business interests and will put those interests ahead of those of the electorate. Add to that a politically correct mindset amongst most people in government, media and the public sector which makes serious discussion of these points difficult without being called racist - and you're home and dry.
