I'm going to give the typical leftie answer: neoliberalism.

Basically, large public works projects still seemed possible up to the early 1970s. The hypothesis is that it's only after two near-simultaneous changes, the rise of Reaganism-Thatcherism and the fall of the Soviet Union, that the state seemed to lose all of its potency. It seems like we placed all of our faith in the wisdom of the market, a machine that can never be stopped once given power.