Tax law and daycare costs.

In the last 100 years rich/educated men are more likely to marry rich/educated women than they were previously. They then are more aware of what it takes to raise an upper-middle class child. Many of these families are smaller and can devote massive time and resources to getting their kid in the Ivy league funnel.

This creates larger barriers for entry for kids who come from families with many kids where the parents can't focus on each kid as much or there is less family income.