They are very sure. There are lots of different things that happened in the late 60s and early 70s through to the early 80s that substantially affected the ec

They are very sure. There are lots of different things that happened in the late 60s and early 70s through to the early 80s that substantially affected the economy. Focussing on 1971 makes it clear that they think only one of those is significant. It's much more effective to let a reader make their own conclusion from cherry-picked data than it is to state the conclusion outright.