Carbonated beverages are not a good choice for meeting an athlete's fluid needs because they

Carbonated beverages make an athlete feel full easily, which limits further fluid intake, and in the long run would result in the athlete being dehydrated easily. Carbonated beverages contain a lot of gar rather than electrolyte needed by the athlete during and after exercise. Imbibing carbonated beverages as an athlete poses serious risk of developing health issues late on in the future which outweighs the temporal benefits the athlete tends to get.