Two reasons:
1) It is unnecessarily complicated to those with no football knowledge. The "tuck" rule, safeties, pass interference penalties when a player is looking back at the ball vs. when he is not, onside kicks, illegal motion penalties, etc, etc, etc.
2) It can be pretty boring to watch for those used to sports like soccer or hockey. Hike the ball, run a 5-8 second play, play stops for 45 seconds-minute, repeat. Add in penalties, time outs, TV time outs & a 3 hour football game only has about 15 minutes of actual action (*yawn!*)
(I say this as someone who has always loved & will continue to watch football).
I find soccer and hockey to be boring. Sure, there's a lot of moving around, but hardly any scoring--zero-zero ties are not uncommon in soccer.
Trying to explain football to a German--in German--is a formidable task.