A motorcycle typically has 2…

A motorcycle typically has 2 seats (for the main rider + 1 passenger).
1 rider out of 2 possible seats represents 50% occupancy which is more than a typical car with 2 people on board and 5 seats (40% occupancy).

Motorcycles do overheat easily (and eventually breakdown) from sitting in stop and go traffic.
The HOV would alleviate this issue by offering a more consistent ride.

Motorcycles in HOV lane free-up space in regular lanes.

The additional "side buffer" offered in HOV lane increases safety.