That's easy, because 1) there is no opportunity for a better life thanks to endemic white racism, and 2) we have a primitive health care system run by heartless robber-barons who let the poor die in the gutters.
/s
That's a characteristic of leftists anyway...totally able to hold contradictory and mutually exclusive positions at the same time. They're able to breathlessly state that on the one hand, we're such a horrible and bigoted country, while on the other hand we better not curtail our heavy immigration. Did it ever occur to them if we were as horrible as they attest, we wouldn't need to worry about immigration?