I have just been looking at early photos of Bell helicopters- the Model 30, which began flying in 1942 shows the TP Floyd Carlson in both the left and right position during early flights, but by 1945 the Model 47 (Sioux) which was the earliest civil certificated helicopter, is almost exclusively piloted from the RH seat as were later models. I suspect it became a case of 'this is the way its always been'.
This tradition is probably more important for landing on ships, which I believe is now always done from the port side of the boat, but no doubt I shall be corrected.
In the same vein, why is the PIC of a fixed wing aircraft always in the left handseat?