I think this post is a great summary of why it's banned https://www.smogon.com/forums/threads/np-ss-ou-suspect-process-round-1-boom-boom-pow.3657265/page-2#post-8303632

Dynamax basically puts WAY too much emphasis on 3 turns, where you either set yourself up to streamroll the other team or blow your only chance of not getting steamrolled. The fact that Dynamax counters itself doesn't mean it's not broken, otherwise we could unban Mega Rayquaza under the logic of "both players can use it, so it's not unfair"
And Smogon has found that alternative fixes besides banning serve as band-aid solutions to mechanics that are inherently overpowered. They tried more Baton Pass restrictions than I care to count before they decided to ban the move entirely.
If you don't like it, don't play by their rules.