Maybe it's a throwback from times prior to runways, when open fields were used- rather like some gliding sites today. I'm thinking that as you are supposed to land on the right of another aircraft (hence vacating left off runways unless advised) it would be wise to overtake on the left on the ground to avoid a collision with landing aircraft. Again I'm guessing, but maybe it is more difficult to remove this rule as there are still some places that still use a field rather than a runway.
KK