why can't we have a registered nutcase system for people that needed even the most minor psych meds and keep them from owning firearms?
Two reasons:
First, a medical provider would be violating the HIPAA Privacy Rule if they disclosed that information, but did not believe the person posed a serious or imminent threat to public safety. That HIPAA rule could be amended, though.
Second, many people believe that your proposal would have the opposite effect on public safety, Namely, people that need psychiatric help will be less likely to seek treatment if they know they'll lose rights or that information will go public. In that way you'd have more untreated psychotic people walking the streets who may pose a threat to themselves or others.