Metal Sonic isn't allowed to speak anymore because words are power, and Metal Sonic is actually incredibly intelligent. He already exceeds Sonic's capabilities, he's already got insane willpower, he's already capable of orchestrating complicated schemes. If he was able to speak, to reason, to goad, to manipulate, it would add a whole new dimension to his threat level, mence, and agency, whole new avenues of what he's capable of doing in this world, and the reason they won't do that ties back into this guy's post >>528569357
Metal Sonic is a big bad in chains because the Eggman is afraid to unleash the monster he created. And the writers are too.