The supply is relatively fixed. The number of NFL seats has grown somewhat since 1990 because there are a few new teams, the season is a little longer, and lots of teams have built new stadiums with a little bit more capacity. But the supply of annual NFL seats is still similar to what it used to be.
But meanwhile, top quartile income Americans earn way (35% more inflation-adjusted), and there are 40% more of them since 1990. This is why the average NFL seat is $170 and the NFL sold out almost every game last year.
You'll notice this in all kinds of areas. Airliners are filling their planes with a greater and greater percentage of first class seating for example. The top 25% of american earners just have an insane amount of spending power.
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