I think it’s because we have so many people from other locales. Not that anyone is wrong or rude per se, but we all have different local norms. For example when I drive in New England I am reasonably confident that drivers will move and cut over. It’s what everyone does, so you know what to expect. In the DC area we have embassies, politicos from all over the US, government employees from all over the US etc. So many different styles of driving you can’t anticipate their moves.