For most of the second half of the 20th century, the USSR existed, China was (as it still is) authoritarian, and the US supported authoritarian regimes in places like South Korea, Indonesia, Iran, and Chile. US-supported regimes killed something like a million people between 1950-1990. Then the 1990s came and the USSR collapsed, liberating Eastern Europe. Also, the US pulled back a bit from its support of the worst sort of authoritarians. So there were reasons to be hopeful during the 1990s. I'd say that on the whole, if there was an anomaly it was more the 1990s specifically than any of the time from 1950-1990. I think the world in general is still less authoritarian on average than it was between 1950-1990.