A big reason is government retired Democrat programs. Ponzi schemes require a growing population, else they implode. Abortion killed off the natural population growth. Third worlders are the only ones left that find moving into a western city nearing bankruptcy a step up.
Thats a really big, probably the biggest, part of it.
Certainly there are other factors at work (Cloward Piven, antiChristianity, guilt over the Holocaust), but the biggest thing is that Western democracies have evolved into these monstrously expensive welfare states. Where due to declining birth rates an increasingly smaller portion of the population support is required to support an ever growing one.
Consider that in the US there used to be 12 people paying into social security for each person drawing from it. Now it’s something like 4 or 5 to one and projected to go to 3 or even 2 to one as the Baby Boomers retire.
So the solution is to import taxpayers to prop up the system.