I was talking with my dad, a big duck comics fan, about this today, I think it's because Disney doesn't want people to associate any names that aren't theirs with their characters. They don't want people to think of Carl Barks' Uncle Scrooge, or Don Rosa's Uncle Scrooge, just Disney's Uncle Scrooge. It's pretty obvious they've been burying those comics in America as much as they can, giving the license to reprint them to Fantagraphics because they know only adults buy from them and not kids.
It didn't work as well in Europe because European readers like to broaden their comics horizons and try out new things even at an early age.
Still, America controls the mainstream narrative of the world, and if duck comics aren't popular there, eventually people worldwide will start thinking they're not popular anywhere.