Culture clash. The Smurfs were originally just a sideshow, much like the Minions today. What's interesting is that the Smurfs world is already quite rich in and of itself and all of the characters are super funny without bringing in more elements from our own world. It's clear that they live more than a few hundred years ago. The way they interact and the various story lines as Peyo put them out probably simply don't appeal all that much to an American audience. Just like the Marvel/DC Comics universes (and everything that gets dragged into it, it's like a black hole) don't appeal all that much to me.