>>The growth in the 1970s was far more than in the 1960s despite the increase in license requirements and the economic troubles. About 33,000 more hams in the 1960s, almost 130,000 - four times more! - in the 1970s. Why?<<
It is a puzzle. Of course there is the baby boom (which you have already mentioned). Many statisticians define this group, to which I belong, as anyone born between 1946 and 1964. The oldest members of this group were about 14 years old in 1960 and the youngest were about 16 years old in 1980. The 1950s, '60s and early '70s when many of us were in High School were a time of worry about the "missile gap" and stress on languages and science in schools -- I am British but even outside the U.S. these historical/cultural trends influenced education. So you had a combination of a very fast growing teenage and youth population (they were building and expanding schools like crazy just to keep up with us!) with a stress on science and technology. Plus, the digital age was still over the horizon, so there was no "competing" lure of personal computing/software vs. ham radio. Back then, amateur radio seemed "leading edge" even to the general population, whereas I have heard several non-hams recently question my re-entry into the hobby earlier this year by claiming that the whole thing is "old hat."
I don't think amateur radio back then was considered "leading edge" by most folks. In fact, just the opposite, once the Space Age and Computer Age dawned. (See below)
But the baby boom data do not explain why, in the United States, the ham population grew a lot more in the 1970s than '60s. The opposite should have happened, because the biggest "baby bulge" came in the years immediately after WWII. About the only (really lame) explanation I can think of is that, in the lingo of the times, ham radio might have been less "groovy" among the politically activist 1960s teenagers than among the recession-hit 1970s cohort.
IMHO, that's a big part of what happened. The counter-culture of the 1960s thought amateur radio was too "square" to be interesting. Also, while the digital revolution was still years away, teenagers in the 1960s in the USA had lots of other interests to compete with amateur radio - cars, electronic music, etc.
Some other factors:
1) In the USA in the 1960s, cb radio was growing by leaps and bounds, and initially probably distracted many potential hams.
2) One of the most-common ways people learned about amateur radio used to be hearing amateurs using AM 'phone on "shortwave" receivers. But by the 1960s US interest in SWLing was beginning to decline. More important, large numbers of US hams had switched from AM to SSB, which typical SW receivers could not demodulate easily. So that method
of spreading the word decreased.
3) In the mid-1960s there were 6 US license classes.: Novice, Technician, Conditional, General, Advanced, Extra. The Novice was a one-year nonrenewable license and the Technician gave no HF privileges at all. The other four license classes gave full privileges until 1968.
Of the hams who had General, Conditional, Advanced or Extra licenses, about 1 in 4 had a Conditional license. That license had the same tests as a General, but was goven "by mail" rather than by exam at an FCC office. Conditional was only available if a ham lived more than a certain distance from an FCC office or had a serious physical disability.
In 1964-65, FCC changed the rules of the Conditional so that the distance requirement was greatly increased - from 75 miles to 175 miles. They also increased the number of FCC exam points. End result was that after the changes there were very few new Conditionals.
4) From 1957 to the early 1960s, the USSR led in the "space race", making most of the "firsts" in space. But once the USA got going, the gap closed, with more and more impressive manned and unmanned successes in space. Finally the USA "won" the race by sending humans to the moon in 1969. In the USA, all this happened live on TV.
With such rapid progress in space, amateur radio seemed to many to be "old hat" even in the 1960s. How could terrestrial HF DX be "leading edge" when we were watching astronauts on the moon live on TV, or seeing pictures of Mars sent back over tens of millions of miles from unmanned probes?
The main point, however, is to challenge the concept that "incentive licensing hurt amateur radio" The numbers show exactly the opposite!
73 de Jim, N2EY