3. Batman is an American exceptionalist.
More than any other superhero, Batman is a reflection on America. He is the only masked marvel whose movies consistently earn tens of millions more in North America than in the rest of the world combined. (Sole exception since 1989: Batman and Robin. Maybe it was funny in Italian.) Action blockbusters typically make about twice as much overseas as domestically. Even Captain America (which was called The First Avenger in some territories) earned more overseas.
If Superman stands for the ungainly immigrant who came here after his people were wiped out in a holocaust and realized his full potential thanks to America, Batman represents the anti-collectivist. He captivates America for the same reason communism never caught on here, the same reason citizens hold firearms for self-defense instead of just hunting, the same reason we are the only country that questions the wisdom of socialized medicine. He is the avatar of a people whose primary wish is to be left alone.