2. Noise regulations. Diesel combustion is loud. Would require water jacketing to muffle noise. Maybe more than already designed it to most.
Did you ever listen to a Harley? Those things can be extremely obnoxious as far as noise. My back yard patio set is 250-yards from the main city street, and at times Harleys going by stops my brother and I from being able to hear each other while talking from across the table (5-feet apart). I can’t see a diesel bike being more intrusive or obnoxious then that.