Pro bike racers use tubulars because they have cars and motos following them with spare wheels in case of a flat. Maybe you meant most pro triathletes don't use them? I don't know anything about triathlon, but I don't expect the pro triathletes have follow cars, etc. like bike teams. So maybe pro triathletes use clinchers.

If I were doing a long distance race and didn't have a follow car I'd sure use clinchers. Wait - I do!