@yogthos OK, I found some info here. All of my assumptions were totally wrong. Humanities increased in the 60s (despite the Sputnik stuff) and then dropped off, had another peak in around the 90s, and plummeted in the last decade.
https://sappingattention.blogspot.com/2018/07/mea-culpa-there-is-crisis-in-humanities.html
(I found the study from a mention in this article: https://blogs.scientificamerican.com/cross-check/why-stem-students-need-humanities-courses/)
@richard_merren I'm guessing that the main factor is whether people see humanities as a viable career, in the current austerity climate people are driven towards choosing careers that are utilitarian