I guess the main reason is the old million mile Harley dealer, Jim Magner, taught me to at 16. He said riding in the left tire track gives oncoming (or passing) traffic the message that you own the lane, just like a car, and they don't need to be thinking about using some of it. Don't know how right that is but that's what I do.
My riding mentor told me the same thing and I practice it to this day.
It makes me cringe when I see some rookie tooling down the highway in the right track.