I was told that fixed-wing captains sit on the left because on Flyer the engine was on the right and Orville or Wilbur was on the left.
Helo captains sit on the right because its generally easier to fly right hand on cyclic and left hand on collective, and in early helos with only one collective (in the middle) that put the pilot on the right for preference.
But lots of former WP designs follow the fixed-wing convention.
Of cours that could all be bollocks.
Sven