Because while the field of mathematics has probably not changed, the field of didactics has changed drastically, and the student expectations in 1980s and 2021 will be also drastically different.
So maybe kids has problems focusing partially due to modern pedagogy? Every word you write down has a cost to the reader, and the less attention span the reader has the more that cost matters. Drown them in too many words and they will just zone out since their attention span didn't last long enough for them to reach the important parts of the text.
As long as you know what a derivative is, all integral and vector calculus is just a comination of “look at the problem with an infinite loupe and then add all those values “.
DiffEq is more or less similar.
Classical maths was very well taught: examples and applications galore.
I've read math textbooks from the 60s onwards. I do not see a trend vs time.