It is true that long, long ago, pre-war, certain airlines (eg American) had right hand doors on their early DC-3s. It was a customer option. But left hand is the norm for the following reason.
Before jetways, prop planes would swing round clockwise in front of the terminal, stopping with their left side parallel with it, and then carry on swinging round as they departed. The captain always sits on the left so this is done this way round so they can judge their wingtip clearances correctly. Believe me, even in a Cessna it's easier to judge the left than the right in a tight spot. Even nowadays some aircraft like the 737 only have one ground steering tiller, on the left of the flight deck, so the captain has to do all the taxiing even if the first officer is flying the sector.
Why does the captain sit on the left ? Because the engine control runs are normally up the centre of the aircraft for mechanical reasons, and as most people are right-handed they prefer to sit on the left of the controls. In the old days such controls were really stiff, you needed your stronger hand.
