I'm not sure if this is appropriate for this board, but I'm curious about sports history and whether this topic is studied and if so whether there's some consensus about the causes behind it. This article in Vox has a neat graph that tracks the popularity of the two sports over several decades of the 20th century.
One thing that should be established is that until the 60' or so College Football was much more popular then the multiple Pro leagues out there, and thus when many schools had to suspend play during WW2 that accounts for the downturn seen in the 40's.
But then when everyone comes back and tons more can go to college on the GI Bill, then all of a sudden you have more players, and more fans, and you see it take off. With more schools competitive, more Bowls popping up, and the spread of more NCAA controlled games showing up on TV.
And with more quality players coming out of college the quality of the pro game began to improve, and of course once the AFL and NFL merged and the Superbowl began it was a perfect made for TV event that could drive viewership.