Because Dynamaxing occurs entirely within the battle and not within the teambuilder, banning specific Pokemon from Dynamaxing poses a unique challenge: communication.
The situation is similar to a Rayquaza with Dragon Ascent not being able to Mega Evolve. To communicate this, a message appears at the start of every Ubers match that states that "you cannot mega evolve Rayquaza".
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(side note: listing things that are stopped in the team validator like evasion seems redundant)
How would you communicate that certain Pokemon are unable to Dynamax? Just saying "you cannot Dynamax certain Pokemon" isn't sufficient, as it doesn't provide enough information. But you also can't list every Pokemon that is banned from Dynamaxing, as that list could potentially take up way too much space. The good option might be to say "you cannot Dynamax these Pokemon" and then provide a link to a pastebin that lists all the banned Dynamaxers for that tier, but even that is a bit unwieldy.
Those comparisons aren't remotely valid. While power and accuracy are certainly desireable traits for an attack to have, they are by no means the only things that matter.
U-turn and Volt Switch aren't particularly powerful attacks, but they're still great moves because of their unique traits. Switching through attacking has a ton of practical benefits such as safely bringing in a glass cannon or getting momentum if the opponent hard switches.
Giga Drain isn't a particularly powerful attack, but it's still a great move because of its unique traits. It lets tanky Grass types stay healthy while still dealing at least decent damage.
Scald is actually moderately powerful attack, but it's still outclassed by other attacks in terms of power. However, it's still a great move because of its unique traits. Burning a physical attacker can be game changing, it can't be shut down by Taunt unlike Will-O-Wisp, and it's still threatening to burn-immune Fire types because it's a Water attack.
And yes, while most priority moves are far from powerful, they're still great moves because of their unique traits. By circumventing the speed stat, they're incredibly useful tools for sweeping frail offensive teams after some chip, revenge killing with a super-effective priority move, or even just getting in a bit of chip before fainting.
That brings us to Dig, Dive, Bounce, Fly, and Phantom Force. Even though most of them are outclassed by other moves in terms of power, they're still respectibly strong, and would in theory be decent options on something that lacked those stronger moves. However, their unique traits make them practically unusable. Even though you're (mostly) invulnerable while charging, it still gives your opponent a free turn to heal or boost, and because it telegraphs itself the next turn, there's little in the way of preventing your opponent from just switching to a resist. And that's not to mention that if a Pokemon has Protect (which is admittedly uncommon in Singles, but still provides some utility in stalling and scouting) then these charge moves are completely harmless.