One other issue with math and science courses is that they constantly build on previous material. You get these long chains of courses in calculus and differential equations and who knows what else where if you didn't master the first one, you're going to have trouble with everything after it.

Getting a D in "British Civil Wars and Revolutions, 1639-1651" won't haunt you for the rest of college. A mastery of lower level European history courses might help you do well in that class, but you aren't in a position where forgetting old material means you don't even know how to start a single exam question.