You only have so many hours to teach in a course, the more kids come in with, the farther you can go. If kids have to be taught what were once taken for granted, something else has to give.
In many countries, such a problem is solved via that a lot of incapable students are "weeded out" by brutal exams (in particular "math for computer scientists" exams are prone for this) in the beginning or in the first semesters - consider it to be a kind of hard "sink or swim"-style curriculum.
