Theory #7 - Caught in the current
So up until now all of the theories have assumed that people drive slow in the left lane on purpose to spite the other drivers around them. This one throws a spin on that a bit. At some point people will move out of the left lane. Oh yes, they will... even if it's the exit after yours. However, there are times where people want to move out of the left lane but can't.
There is a phenomenon that happens when people end up in the left lane but don't intend to stay there, or realize too late that they need to move. When drivers are slow enough the quicker drivers take any chance they get to move around them. If enough drivers start passing they effectively block the exit route of the offending left-lane driver. No matter how hard the driver tries to move over they can't because the quicker moving traffic keeps moving around them. Compounding the problem is that drivers caught in this situation are typically not very assertive. That means they can't move their way into the traffic until someone decides to give them a chance again. Should they wait too long they will remain trapped on the left edges of the highway.
Maybe I've been on the East coast too long, but if you are driving on a freeway you have to be assertive. There's just too much traffic to be wishy-washy about what you are going to do. I'm not saying to drive aggressively...there's a difference. If you're a person who has a hard time making decisions (like which exit you are supposed to take...) maybe you should think twice before venturing into the left side of the road.