Because they get 200 mpg- noone has one- something a few dieselhead buddies of mine have been discussing. Oh, here's some picts of a British rally last year-
http://www.dieselbike.net/BritishRally07/Britishrally07.htm
and of the Enfields- "Royal Enfield in India was the only manufacturer that has built a diesel motorcycle in mass production. They put a 6.5 hp industrial diesel, in the frame of the British-based Royal Enfield. Because of pollution laws this bike is no longer produced. It was one of the most fuel efficient Bikes in India returning over 200 mpg."
http://www.peace65.freeserve.co.uk/Pictures/royalenfield.htm