It's an idiotic law, and that's part of the reason. "Left Lane = Passing Lane" works just fine until the highway starts to fill up. Then it becomes impossible, and the high-speed idiots who don't comprehend reality start weaving, tailgating, and causing crashes. They should be shot.
Just because it doesn't always work doesn't mean anything. There are also minumum speed laws but no one expects them to be enforced in a traffic jam.