I've always wondered: Why can't some schools boast large AP programs as other schools do?

Seriously, is this a real question from a “resident” of the Windy City?

My son’s GF teaches in a Chicago Public high school, where the most of the Frosh are struggling with (remedial) middle school math. The idea of taking AP Math is a non-starter, if they don’t have the basics of Alg I coming in