Because if plot goes different way with AFO winning, whole world would be changed entirely, and with that happening MHA has chance became Great Series. But opportunity is lost, and now when same thing is going to do Shigaraki, he will lose too, case this is final arc. Author destroyed whole tension of this series because he understood that if he write down AFO winning fight, that will bring challenge for him. So he decided chose peaceful way of plot. This was moment when I lose faith in this series.