“Too Much Calculus” by Gilbert Strang, who taught me linear algebra in the late Victorian era and is still at it. Linear algebra is what we use for everything in the real world. Calculus is elegant, but you’ll never actually have to solve an integral. http://www-math.mit.edu/~gs/papers/essay.pdf …
I don’t know; I wasn’t paying attention to my classmates’ difficulties at the time. I gather that most people want to see concrete examples and methods first, and principles later or (preferably) never.