I'm not an agent, my mother is in the insurance business, and she couldn't tell you why they charge what they do other than, that is what they charge. She encourages me to shop around becuase when they start losing policies is when they lower the rates.
I assume State Farm could careless whether or not they insure motorcycles. Geico on the other hand specializes in it and hopes to fill that nitche of the marketplace and make a buck. State Farm would rather sell you life and home owners insurance. With American Family their high risk side, American Standard are the only ones that write motorcycle policies.