Explained in this interesting and concise video:
In reality, human beings evolved to have ethnic identity and ethnocentric cooperation. This is the real basis of socialism as a political or economic form of political organisation within a modern nation state.
Multiculturalism and multi-ethnic mass immigration will undermine and destroy a society that has a strong level of social and ethnic cohesion with democratic “socialism” in the non-Marxist sense (that is, a welfare state, generous social security, universal health care, and interventionist government policies to promote the common social and economic well-being of the community).
The modern left is too stupid and deranged to understand that (1) ethnic nationalism must be the basis of progressive liberalism, old-fashioned Social Democracy, or democratic socialism, and that (2) the Cult of Diversity is, ultimately, a death sentence for economic and social progressivism or socialism that many on the Left support.