Why are the passenger doors on the Left side ? - because it's an old nautical convention handed down over the centuries and passed on to aviation.
The Nautical convention was to dock ships with the Port, i.e. the quay, on the Left side, hence this became known as the Port side. The brothels, with their distinctive Red lights, were lined up along along the quay, i.e. the Port side, thus the position lights on the Left / Port side of the ships were Red. Don't know how the green lights came about, sorry.
It seems that the Russians didn't follow that convention, their brothels must have had Green Lights.
Regards,
Old Smokey