The U.S. Supreme Court has held that under the 14th Amendment the state need not provide a jury in a criminal prosecution if the maximum possible penalty for the offense is six months or less in jail/prison. Duncan v. Louisiana, 391 U.S. 145 (1968). Since most traffic offenses do not involve much, if any, jail time the state is not required under the Constitution to provide a jury for such offenses and most states do not do that, though a few do. The primary reason for that is that jury trials take more time and cost more, and citizens dislike jury duty enough as it is already without having to sit on a traffic case (which most people, I think, would feel is a complete waste of their time).

