A big part of it is the fact that in order to teach an AP, the teacher has to take the course over the summer. And has to have the degree. Plus, depending on the school, prerequisites are required. It’s just different from school to school. I know that my school-which is nationally ranked as well as named one of the hardest schools in the country-has a policy that doesn’t require prerequisites, and has many AP versions of regular classes, which are taken by the majority of students (for example, the APUSH course has about 4 classes (just for juniors mind you) where the regular one has one small class. So some schools might deem an AP program not worth it-if it’s not a popular class.