I do not think the graph applies solely to America or that the stagnation or even fall of the share of gains only happened in US. I think much of the west shares the same story, thus, reserve currency, unique place in the world after the war etc explain the large gains in power, but not the effect of the graph as if it were so there would be a different story for Europe which there isn't. Therefore, it probably was the Thatcher Reagan policy changes.