1. That was a period of super-high growth. Seeing positive change is one's life is one of the greatest contributors to happiness (one easy way to tap into this mechanism would be to have a terrible war that destroys everything, and then just have get their lives on track again).

2. Culture changed. In the past, people were told by society to not complain about their mental state, whereas now people are opened up almost to the point of being exhibitionistic. So, you cannot really compare reports from 50 years ago with today's numbers.