Not all do the Bell 47 for example was designed to be flown from the left, and especially in long line lifting, sitting in the left is preferable as you can lean out more easily to see the underslung load........but, in general:
The collective is in the left hand the cyclic in the right. The collective requires less attention during most phases of flight, whereas the cyclic requires almost constant attention.
The collective because of the balance of forces on the rotor disc should remain where you left it, if you let it go, whereas the cyclic in older helos especially will often just full over!
So, now you want to switch radio frequency or adjust the QNH etc, which hand would you prefer to use? The left!
Bear in mind that many helos have simply one centre console for instruments & radios and that larger more modern/complex helos often have a centre pedestal between the pilots.
Thus the right hand seat is most prefered (but not always most useful!)
Why do fixed wing pilots sit on the left?