Tradition, ya know...Doesn't controversy sell?
I'm of the belief that sports in general and football, pro football, in particular, suffers from over saturation. The NFL went from one day a week, to now 3...and after the college season, 4 days a week. There is also the standard concern over the idea that there are simply too many teams, diluting the player depth as well as having the league need to support teams that fail to draw. Franchise moves will also upset the fan base. Is it better to have 60K in St Louis or 30K in LA? I dunno...
While I well understand the military aspect of football, so perfectly dissected in Carlin's routine, the ever more present, growing and ongoing association with the US military might also have detractors that have helped cause fewer eyes drawn to the sport.
Health concerns also have given rise to a new sensibility. Perhaps it has just run its time.