I have noticed a tendency for many slow, timid appearing drivers to prefer to campout in the left lane because it gives them the least amount of interaction with other highway traffic.

On a typical 3-lane highway, the right lane will have many big trucks, plus merging traffic entering and exiting constantly. The center lane puts them right in the middle of all the chaos, getting passed by other traffic regularly on both sides. In the left lane they can set their cruise control to 5mph below the limit and squat.

Even when there are lulls in the amount of traffic, many times I have approached this type of driver from behind in the left lane, paused to see if they would move over for me to pass, and they almost never do.

Disclaimer: I like to drive 5-10mph over the speed limit :)