Because if we're in any other lane, I will pass on the left. I will not pass on the right except when absolutely necessary, because that is dangerous. It will leave me in your blind spot longer, and it increases the chances of two cars trying to merge into one lane at the same time. Surely you have seen this. It can be avoided by minimizing the need to switch lanes to the right.
Traffic flows better with slower traffic keeping right.
How slow you go should indicate how far to the right you should be. And this speed, by the way, is relative to other cars. Not the speed limit signs. If you are the only person on the road trying to maintain the speed limit, then you should be in the far-right lane. This should be a no-brainer.