Price elasticity of demand, pure and simple.
Demand represents the inverse relationship between price and units sold. Therefore there are two revenue points on the demand curve that are equal (6 units at $9 = 9 units at $6 = 54). The optimal point is somewhere in between. (7 Units at $8 =56 > 54). Extrapulate that over 35,000 potential customers at any given event and that's a pretty penny.
You are correct that a certain number of people will continue to shell out money for concessions at football games. (whether for themselves or for their kids, whathaveyou. we do not have to limit this to beer). They represent the 6 units at $9. The vendor will not lose these peoples' business. In fact if prices were lowered, the vendor may get a couple more purchases from the same sample. So instead of selling 6 units at $9 to 6 people ($54), they sell 9 units at $7.50 to those same 6 people ($67.50). That's a 25% increase by dropping to a still inflated price compared to what one can purchase at Crazy Bruces' or even Margharitas' down the road after the game. The question becomes at what price can the vendor sell its product to maximize total revenue, but I highly doubt it is currently optimum. Many of my tailgate group and a good deal of friends do not purchase concession there simply because of price. The primary tenants are college or minor league teams, so concessions are not used to pay entertainer salaries like at Fenway or Yankee Stadium. It is thought to be a form of price gouging.
That is one part of the problem. The other parts involve stadium management and quality of entertainment. This is why I hope Global Spectrum can begin to remedy the situation. It is on them to make the venues more attractive and book more dates around the primary tenants. It is on them to create a family friendly atmosphere and attract more visitors (not just for football, but for everything).
Compare and contrast this situation with New Britain Stadium. Now admittedly I haven't been there this year (I am planning on going on Saturday), however their concession prices are still quite a bit lower. Last time I was there, my friends and I drank $5.50 Sam Adams all game long. The stadium seats about 7,000, the max ticket costs $12, and it is not always sold out.