I can't speak for all North Americans, but I think part of the reason why soccer hasn't caught on here is that its pace is seen as boring. It's not just the scoring, but the whole speed of the game. Watching players bring the ball up through midfield, and watching them take time to set up plays in the other's end, is a little too subtle for many here, in my opinion.
Part of it is a cultural thing, and some has to do with the other sports that have already gained a foothold. We tend to like sports that move very quickly from end to end like hockey and basketball, or those involving a lot of speed and violence like football. North Americans, in general, aren't exactly the most patient and leisurely people in the world, so watching soccer is sometimes a difficult thing for people.
Your comparison with NFL scores using touchdowns doesn't really work, because Americans love the action that precedes a touchdown. A big run, a great reception, or a big hit - whatever it is, they're always in the game. In fact, Americans love even a low-scoring football game because it can still provide plenty of excitement. On the flip side, they can't understand how a 0-0 soccer match could possibly be exciting, because only goals seem to get people excited about soccer here.
Soccer is a great sport, and I'd like to see it catch on more here. I had a chance to attend some of the U-20 games here recently, and I was encouraged by the turnout. However, it's going to be a very uphill battle for it to gain a foothold in North America.