Casual fans get footballed out. My candidate is the movement to smaller markets. The Rams went from Los Angeles to St. Louis. The Raiders from LA to Oakland, the Oilers from Houston to Nashville and the Cleveland Browns became the Baltimore Ravens. Expansion teams went to Jacksonville and Charlotte.
Smaller markets mean smaller audiences.
The first year Los Angeles had no team, pro football TV watching fell about 40 percent. Here in Houston, Oiler games in 1995 had a 19 rating. Last year, the Oiler games carried had a 10.6 rating. One thing keeping Houston attuned to pro football on TV is the popularity of the Dallas Cowboys, which tends to build interest. Now there's no mystery about why NFL teams are moving to those smaller cities.