This could probably be debated for hours, and I’m not firm on this belief (curious to hear what others think) but I believe Reagan contributed to this decline with the deinstitutionalization of mental health.

With the slow ascent of wealth inequality causing more people to feel financial strain, people at the bottom who mentally break no longer have a means to get mental health care.

(These days it is incredibly hard to get mental health care in the USA regardless of your financial status, IMO.)