The 60s were when due to “activism” and court rulings it became much harder to involuntarily hospitalize people for mental health reasons, and most state mental hospitals were shut down. After the 60s is when the numbers spike.
Causation, or just incidental correlation?
They types of people mostly known as serial killers were NOT the same types of people involuntarily committed before the 60’s for mental health reason. Efficient, covert serial killers, those wily enough to avoid capture for extended periods of time, are highly functioning individuals, many of whom can blend in with society sufficiently enough to avoid detection. These would not have been deemed mental institution material by pre-60’s standards.
Consider Dennis Rader, Kansas’ BTK killer, who was a productive family man, church elder, boy scout leader, and supervisory city worker.