>>58733868
Because Lysandre firing the Ultimate Weapon was canonically the closest the world's ever come to being destroyed at the time of XY, therefore it necessitated Zygarde's intervention.
LZA once again proves the Ultimate Weapon's threat level by not only requiring Zygarde's intervention, but also requiring it to engage in prep time by seeking out its cells and a worthy trainer to help it achieve Mega Evolution, therefore implicating Ange as an even bigger threat than the original Ultimate Weapon because it requires Mega Zygarde's power to deal with.