>>54825387legendaries raise the skill floor too high for many non-legendary mons to be able to compete, not even mentioning how they warp the tier's optimal compositions and strategies around their presence
so they not only outclass, they also dictate
you can play around legendaries with a smattering of underpowered mons as options, but you blow your options up so much wider by banning all legendaries, which were never intended to be bromons anyway, and battling with bromons the heart of the series's experience