In the Muslim scriptures from what I read on the internet, gambling is categorized as a sin because it contains elements of cheating... logically, we all certainly hate and condemn all cheating done by bookies to get money from gamblers... that makes sense and may be another reference, why gambling can be categorized as a sin, one of which is to avoid other cheating.
Until now in football matches or other sports, we often see that players or athletes are threatened by gambling mafia and are forced to give in for the victory of other teams or athletes, this fixing score often occurs and reduces sportsmanship in the world of sports because of the motive of cheating by gambling mafia.