BSA Membership Graph (1911 - 2023)

Stolen from Reddit.

The big drop in the 1970's was supposedly due to a change in the program to de-emphasize outdoor activities. The step down in 2019 was the LDS church cutting ties and starting their own program.

If you consider this as a proportion of the population it's an even bigger drop. In 1970 there were about 4.8M scouts in a population of 205M, so about 2.3% of all Americans were in Boy Scouts. Now it's 1M scouts in a population of 341M, so only 0.3% of Americans are in Boy Scouts.