Why do regressives want to saturate the USA with third-world culture? ?

Yeah, those christian white supremacists need to go. Then America will no longer be a third-world country.

You mean with highly religious people who don't pay taxes and are used to violent, dogmatic authoritarianism? 

I wouldn't refer to liberals as "regressives".  However, you have to understand how liberals arrive at the conclusions that they truly believe.  They see the world through the lens of emotion rather than through dispassionate objectivity.  In addition, they assign emotion to the conclusions that conservatives make objectively--"conservatives HATE the homeless".  As such, liberals do not consider the impact of their policy--only how the policy makes them FEEL.  It is a feeling of compassion that leads them to coddle those who do not contribute to society regardless of how that affects the quality of life of society as a whole.  Countries that suffer from leftist political thought (Cuba, North Korea, Vietnam, Venezuela, etc.) are dealt with using cognitive dissonance to satisfy emotional thinking.  

Minorities won't assimilate, improve themselves or aspire - so the whites must be pulled down to their level.  Nostradamas predicted it.