Not because anything fundamental has changed about economics, but because baselines have shifted to the point that what we expect an efficient market to look like may be very different from what what people expected it to look like in the early 20th century. So, basically, people's definition of "efficient" has subtly changed.
A century or so ago market economies hadn't caught on to the same extent. In the 1950s you had Khrushchev coming to visit Iowa to learn about US agricultural productivity. He visited family farms, talked to people about how they did things, and then went back home to tell the USSR's farmers that they needed to plant corn everywhere, including in places where the climate is not even remotely suitable for growing corn. All this time talking to family farmers about how they make their own decisions about the best use of their own land, and he somehow still failed to pick up on the idea that maybe the secret ingredient in the sauce was that the USA generally let farmers run their own farms.
Sure, the USA's capitalist economy still had charlatans, including agricultural charlatans, and wasteful fads for bad ideas, rent-seeking behavior, pork barrel politics, etc. But maybe it was still easy to see that situation as efficient at the time, because one's reference for comparison included being able to see the greater amount of damage that a planned economy allowed a charlatan like Lysenko to do from his position of power within the Soviet Academy of Sciences.