White people are on average will have more socioeconomic freedom, so less reasons to walk around in a band or associate with anyone for survival value. This naturally leads to drop in birth rate.

Poor people have more reasons to band together, pool their money to afford nicer things. And they've more reproductive pressure because of this reasons, so they are bound to grow population wise. This is seen nearly everywhere even in places where there are no good social weflare programs like India.

It's a curse that comes with freedom, individualism and prosperity - you've end up with no reasons to reproduce or find it very difficult to associate with someone because it seems so limiting.