This is not against NASCAR drivers, and I agree to say NASCAR drivers are not inferior to F1 drivers. NASCAR and F1 (or more generally open-wheel) are too much different to make a fair comparison. A good open-wheel driver will not necessarily make a good NASCAR driver, nor a good NASCAR driver will necessarily make a good open-wheel driver.
FIA and Formula 1 officials just don't want to see too young drivers coming directly to F1 anymore without having enough open-wheel experience.
Open-wheel drivers usually don't come directly to Sprint Cup. They often enter some ARCA, Xfinity or Truck Series events before that.
Formula 1 is considered to be the top of international open-wheel series. But before coming to the top, you have to gain experience enough at lower levels. This is what F1 officials are aiming to.
And before you whine, note that I wrote "
is considered", because I think IndyCar Series is at the same competition level as F1, just not as technological as F1.