NFL is probably the most popular US sport, so I have never figured out why there is no minor league. There are certainly plenty of college players, and people love the sport itself. Heck my local college, a pretty poor Div I team (UConn), is able to pack in 50K+ at their stadium.
Maybe the NCAA wouldn’t put with the competition.
I think the reason for that is the shortness of NFL careers, the average NFL career is only something like three years. So if a player spends only two years in the minors, they would be damaged goods by the time they would finally make it to the NFL.