The engine mounts allow some movement in relation to the hull. With a hull-mounted compressor, tension on the belt would vary with engine movement. Belt tension also might throw off your engine and prop-shaft alignment.

This opinion is based on my experience: The first time I ever attempted to do an engine alignment I was sitting on the engine while making adjustments. Needless to say GOBI was at work and the result was not even close to perfect. I had to re-do it after I realized why I couldn't get consistent results. It's only logical your compressor belt would have the same effect.