College football was huge in the northeast and midwest going back to the early 1900s. Walter Camp, the coach at Yale, invented the modern football field and set the rules that created the modern game. Pennsylvania and Ohio were semi-pro and pre-NFL pro football hotbeds, beginning at about the same time. The original National Football League was around for a couple of years beginning in 1902.
The modern NFL began in 1920. Red Grange put them on the map, beginning in 1925, after he had been a legend at the University of Illinois. George Halas had also played at the U of I a decade or so earlier, and played pro ball in the Chicago area before setting up the Decatur Staleys in 1919 -- today's Chicago Bears.
College football was more popular than the pro game, at least until television era. NFL teams assembled regional networks of stations to air their games, with most of them airing on Dumont affiliates.