There's so many factors that go into winning a football game, that the NFL has special rules for a lot of things in an attempt to level the playing field so that the only advantage a team has over another is their coaches and players. One of these measures has to do with the way stadiums are built.
The NFL rules dictate that stadiums should be built on the North South direction. This is so that sunlight doesn't play any factor in the game. That way, a quarterback can't complain that they weren't able to play well because they had the Sun on their eyes. There are 7 stadiums not built East-West, but they either took measures to make sure that the sunlight doesn't hit players, or they're indoor stadiums, in which case, direction doesn't matter.