Why is it so hard for people to keep right on the Parkway?

Cape May - Why is it so hard for people to keep right on the Parkway? This is something that is in the driving manual that is supposed to be common knowledge, like stopping for stop signs or red lights. I am sure that this is the cause of many crashes. All folks have to do is to drive in the right lane until they pass someone, and when they are in the left lane when someone comes up behind them, get over to the right.