There may be any number of reasons but perhaps key reasons have to do with the size and valuse of a bike vs a car and seasonal use considerations. Let’s talk about size. A motorcycle can certainly be substantial, but it is in general much lighter and has less mass when compared to a car. As a result, the relative damage that is could do to another vehicle may be less, which can therefore effect the liability pricing. As for physical damage (comp and collision) here too, in a great many cases, a car may carry a higher value than a bike, thus effecting the comp and collision picing. Finally, there is the seasonal aspect. Here in CT for example, we cannot ride 365 days out of the year like you lucky riders in warmer states can. Thus, because our bikes are on the road less, insurance companies may charge less because of the decreased exposure.