Because it is about rewarding good behavior that maximizes a player's potential. Look, if Jake Thomas is outperforming JJJ in practice...if he is playing to the game plan, understanding his role defensively, etc., starting JJJ over Thomas because of "potential" doesn't do Marquette or JJJ any good in the long run.

Furthermore I am not convinced that playing JJJ or Dawson would really improve MU's chances of winning anyway.