I can't say I know, but 4 guesses would be 1) Harley didn't want to pay for product placement, 2) Maybe Henry Winkler was just learning to ride and couldn't handle a larger bike, or 3) They thought a smaller bike looked friendlier for the cameras and character or even 4) The Fonz was a wussy. Probably not that last ne since I still make jokes involving hitting machines to get them to work.