I've worked in colleges for 2 decades. It's rarely purely academic issues that cause bad grades, in my experience.

It's everything else with self regulation. Where most college prep goes wrong is at home, in not allowing appropriate agency for the child to make decisions before college. When you go from having to ask to go to the bathroom to being entirely independent in 2 months, bad shit happens.

This lesson has held true everywhere I've worked, from small junior colleges to D1 state schools.

Subsequently, college has gotten harder, but so has k-12, so proportionately, no it's not more difficult now