2. Player Health
Fact: PBS counted 147 concussions in the NFL in 2013, based on weekly injury reports.
There were almost certainly many more that went unreported. For many football fans, revelations about concussions and long-term brain trauma (see: League of Denial) have made it harder to enjoy the sport. Football is well-designed, complicated and can be played successfully with a wide variety of strategies, but that's all overshadowed by the violence. ESPN reported concussion rates in high school football as nearly triple those in soccer, basketball or baseball.
As a fan, it's impossible to encourage other human beings, especially children, to risk their bodies and minds for the sake of our entertainment. Soccer has all of the tactical discussion and chess-match back-and-forth, with the added bonus of a much lower chance that a few years of the sport will ruin a player's life.