You should consult with a lawyer.
But as far as I understand (and I admit I don't know much about the subject), in the USA there's no law that requires a platform to provide that kind of protection, so what you're asking for is something that doesn't exist and doesn't make sense.
There are laws for educational institutions, but not for platforms like Itch.
Depending on the state where you live, there might be laws that allow you to report the aggressor. Even so, I return to my first piece of advice: consult with a lawyer, because often what you consider bullying might not be considered as such under the law, and you could be exposed to a countersuit. So don't trust what you read on the internet and consult with a legitimate lawyer in this area, who can advise you.
My personal opinion, but threatening legal action against a platform, instead of taking legal action directly against the aggressor, seems pointless to me. But again, don't ask online; consult a specialized lawyer to avoid getting into any legal trouble that could cost you a lot of money if handled incorrectly.
Why do itch do nothing about reported bullying?
Regarding this question, two things could be happening. Itch's staff is small, and they may not have reviewed your report yet.
Or, they reviewed it, and the evidence you presented isn't enough to prove you're actually being harassment.
Itch doesn't usually respond; instead, they analyze the situation and take action if necessary. If you're expecting an email saying you're right and explaining what they're going to do, that's not going to happen.
What might happen is that they delete any messages where you're being threatened and block or ban the other person's account, but they won't send you an email notifying you of their actions.
EDIT:
Since harassment is a serious issue, I recommend going to the help forum, posting your ticket number, explaining the problem simply and briefly, and politely requesting the moderator to inform a staff member about your support ticket.