It's not so much that it's esoteric, it's that it's not something most people use every day, and skills decay over time. I mean, Differential Equations (Diff-e-screw) was a year 2 class for me, and I assume I'd fail it if I took it today.

But most people that have any exposure to it can pick it back up fairly rapidly. And most people that have a reasonable exposure to math in general could probably come up to speed (a little less) rapidly.