Part of that is just math. Football teams have much larger rosters than basketball teams. If you limited your search to only defensive linemen, then you might see a lot more football games without NFL prospects.
The stars in football are usually skilled players so let's limit this to them. It's rare you see a D1 game with no QB, RB or WR who could make an impact in the NFL. Making them stay in school till 21 is a big part of it though. However there were many surefire NBA stars in college even when kids could turn pro out of high school.
Just seems like a lull in talent...