People have been steadily getting poorer thanks to inflation eating their stagnating wages combined with skyrocketing consumer and housing prices and the birthrate keeps dropping.

Poor people had plenty of kids because they had no standards nor accessible birth control or they come from a conservative religious culture where having kids is the norm.

But once people taste the good life, like westerners had it so good a while a ago, they don't want to bring kids in economic conditions worse than before, so the poorer you make them, the less kids they'll have.

So to compensate, you don't focus on improving the conditions for the locals to convince them to procreate, but you open the immigration gates to people from poor places with no standards, happy to bring kids in conditions that are way better than what they have in their own country, even though they're worse than the locals had a few decades ago.