It could also be caused by authors getting better at expressing themselves succinctly.
There may also be something about people not wanting to pay "full price" for a "thin book" but I doubt that's a major factor.
I did feel I was stretching to fill some pages in the first edition. That said, I was asked to do a second edition and it actually ended up a bit longer. I cut some "filler" that I didn't think was essential, reorganized, and dove more deeply into some topics (like legal matters with the assistance of a lawyer colleague) and I think it is now reasonably in right length territory; it covers a lot of territory.
Another book I self-published on software packaging broadly and its analogs to various things in the real world today and historically ended up around 100 pages.