Demographics.
I went to university just about 10 years ago.
Math classes had fewer and fewer people as we escalated up.
College Algebra, Trigonometry, Pre-Calc, Calc I, II, & III, Differential Equations, Abstract Algebra, Boolean Algebra, and engineering courses which were Math based.
The classes were vastly white male, with a group of international Asian students.
Same. I was in college over 20 years ago, and as I went up the ladder, the demographics were starkly different.