The AP Exam covers Java and you have less than 180 days to cover everything from "what is a variable" to inheritance, recursion, and sorting algorithms.
My school has gone back and forth with starting with Python or Java in the Pre-AP course. There are pros and cons for both options, but ultimately a language is just a language. The concepts are what is really being taught.
The difference between a math variable and a computer variable and how "x = x + 1" works can be difficult for some students to wrap their minds around. Getting students comfortable/competent with a language and then switching it mid year would be a tough ask for quite a few students.
Keep in mind, many of the students taking Computer Science in high school are not exactly the same students that would have taught themselves to program at 15 and 16. Self-taught students are easy to teach (although they provide their own challenges), but you have to approach things much differently (and often slowly) for students that don't already "think like a programmer".