As Tim said.
Helicopters are dynamically unstable, and 99% do not have an autopilot, so the driver must hold the cyclic at all times. The collective can be frictioned to stay where it is, which means the left hand is available to change radio frequencies, move heading bugs, scratch yourself.
When flying from the left seat, it is a PITA to keep swapping hands to change the radio or punch the student.
