Why is the PIC position for helicopters the right seat, rather than the left seat as in airplanes?
The reason is both historical and operational.

The first mass produced helicopter was the Sikorsky R-4.  It had a single collective located between the two pilot seats, so by necessity, the person on the right would control the cyclic with their right hand, and the person on the left would control the cyclic with their left (because their right is needed on the collective).  The cyclic in the R-4 was very difficult to manipulate and required a lot of strength and coordination.  Since the vast majority of student helicopter pilots were right handed, they sat in the right seat, allowing them to use their dominant hand on the cyclic.  Thus, the first generation of helicopter pilots all sat in the right seat, since they all learned on the R-4 (or the R-6).  That's how traditions begin.

The operational applications of sitting in the right seat relate to the inherent instability of a helicopter. A helicopter pilot rarely wants to let go of the cyclic, as it often requires continuous control inputs, especially when hovering. The collective, on the other hand, does not need to be monitored as closely and can be held steady with a friction adjustment, so that hand is occasionally free to manipulate things like radios and other equipment, which are located in the center of the cockpit.  

Most helicopters these days have a collective for each seat, located on the left, so the pilot wants to use the hand in the middle of the cockpit to work the radios and things.  Hence, they sit on the right side, which places the collective hand in the middle of the cockpit.

You can read a longer article about this: Why do helicopter pilots sit in the right seat?