The Manufacturer Decides
Guy's and Gal's,Guy's and Gal's,
Eh Eh Eh, Surely (and dont call me Shirley!!!) the reason is, because that it is the way the manufacturer intended them to fly the A/C when launched.
However.......Subsequent mod's and upgrades, whether changes to airframe or drive train, would not have changed this if it were not for legestalistic(legal)(Spelling(Help)) changes/reasons.
At the end of the day if the A/C is to be flown with both seats occupied then the Capt will, most likeley sit on the left, however if it is a single pilot A/C (ie Bell 212) then the Capt will Probably sit on the Right.
The simple reason is controllability(ergonomics...for those with degrees!!)
Cheers
L'