Well those are research papers, which have a different purpose, and I suspect in some fields research papers are actually more inscrutable now than then. I'm talking about books intended for an audience like the linked one, high school or early undergraduate. I haven't done an exhaustive survey or anything, but most of the ones I've looked at on calculus, trigonometry, etc. just wouldn't work for most students now; with the improved accessibility, much higher numbers of more diverse students are learning this kind of maths.
