Classes required for the major and those that meet general studies requirements are generally offered more regularly. Upper-level classes and electives are on rotation, depending on faculty availability and preference. That means one faculty member might have four or five (or more) elective classes that they teach (and that no one else teaches), but they have to rotate them with more of the bread-and-butter classes that they teach more often. In big research universities (or even elite LACs where faculty are expected to engage more with their research), teaching loads are lower, and that means a professor might teach a particular class only once every few years. Even in a more teaching-intensive university like mine, the same could be true (next semester, I’m teaching a class that I haven’t taught since 2015!). Because those elective and more specialized courses might be offered by only one professor, it’s always a good idea to ask faculty when they think they might next teach a particular course. It’s entirely possible that the course might not show up on the schedule again until after an interested student has graduated.