I have a theory that football and basketball never developed minor leagues because they are the two major sports whose collegiate leagues were more popular than the professional leagues through the middle of the 20th century.
Not a theory at all. You’re 100% correct in the case of football. It’s a little more complicated in the case of basketball because they sorta DO have a minor-league system, the Development League (G League). College sports make such well-trained stars that the majority of basketball players (but no longer a super-majority) skip D League.
Those sports can’t develop effective minor leagues because they lack baseball’s antitrust exemption. Teams aren’t going to pour money into a minor league team when they can’t retain the rights to those players.