Earlier quoted context omitted.
Not comparing apples to apples, though. Those government bonds were, by any reasonable measurement, risk free (EDIT: as another commenter noted, not exactly, we could call them "minimal risk"), while "the market" is not. Looking back in hindsight is always risk-free, though, which can lead to faulty conclusions.
Exactly, if it had been obvious at the time that "the market" would deliver a better return, for certain, then nobody would have bought bonds at those prices. Then bond prices would have declined (and their expected returns or interest rate would have increased) until, in equilibrium, the anticipation was that the stocks and bonds would deliver comparable expected risk-adjusted returns.
A more interesting graph would be to show me the 30 year return at each point along the way. My gues is that stocks would still mostly come out on top, but not the runaway you see here.