IIRC, one of the things that changed back then was a movement away from workers having retirement plans with the companies they worked for (pensions) and toward managing their retirement individually (IRAs). This could have conceivably led to a new, steady source of investment in the stock market through mutual funds and whatnot (and perhaps even more aggressive investments presumably), which, if the changes shown on that graph could be thought to be generously helped by investment income, may explain some of this.

Disclaimer: I don't know enough about this stuff to make a definitive argument.