Ok, here comes a semi-educated guess, but I do think I've heard it somewhere...
The command seat is on the right in helicopters out of old habit (large helicopters) and ease of construction (SP helicopters). With the pilot to the right the mechanics involved to get his inputs to the rotor are shorter and simpler which also equals less weight. You save a few feet of push-rods and might use the space for something else. But since most helicopters today have the option of dual controls, it's back to the old habit reason. And not to forget: Plank-drivers have the command-seat to the left so helicopters must be different (also old habit).
Another way that helicopters are different is in that the pilot-seats are sometimes comfy in airplanes, but that's another thread...
/2beers