For lots of students, Calc II is "death by a thousand cuts". Unlike earlier math concepts, calculus (at least, as generally taught in the US) requires recursively hammering away at a problem along MANY steps that generally require a lot of time and paper... often, with no real confidence that you're even on the right path, and seemingly infinite ways to screw up somewhere along the way. Often, you don't even get the satisfaction of closure... you either just run out of time, or run into a sufficiently bad dead end that you can't figure out how to get past it.
Part of the problem is that math teachers are painfully aware that we're never more than 3 or 4 generations away from losing advanced math and descending into barbarity. The current generation of mathematicians can deal with AT MOST one or two lost generations of students before there's a risk that advanced math will die with them. So, they force almost EVERYONE to learn Calc II as a potential mathematician instead of someone merely taking advantage of a useful tool.
I believe that at some universities, there's a tense compromise between the math department and everyone else whereby students in majors that need to be able to USE calculus are required to take full-blown Calc II at least once... but if they fail to pass it with a sufficiently high grade, they can retake it as an alternate class that settles for making sure they know how to USE Calculus.