Before circa 1970, it was more common to say "kilocycles" than "kilohertz" (and analogously for "mega-," "giga-," etc.). What contributed to "Hertz" becoming more common than "cycles"?
SI units started to be used.
In 1960 (not 1970!), the Système international d'unités was established at the General Conference on Weights and Measures. The hertz was officially recognized as a unit derived from metric system units (namely, the second). It was already in partial use (the International Electrotechnical Commission began using it in 1930). With the rise of the hertz, the phrase "cycles" in "cycles per second" fell out of fashion, leading to the general trend we see on the Google Ngram.
Interestingly enough, adding "hertz" to the Ngram search shows that its usage, too, took off a bit after 1960, though it was already fairly popular.
The General Conference on Weights and Measures (1960) adopted the herz unit. So you have found that it took about 10 years for English-speaking books to catch up with that change.
There was indeed some resistance to adopting the new name in the United States. In 1967, Rufus Cartwright wrote about it in the magazine ``Electronic Illustrated":
Is the short, illustrious life of Heinrich Hertz any reason to change the English language? A quick answer is no, especially since hertzian waves and antennas already honor him. But that may be the short view. Perhaps we must take the long view and see life as a complete hertz.
What might have helped in popularizing the new unit is that saying 'cycle' is potentially confusing when 'cycle per second' is really meant.