I would assume because they are both offered in the same period.
I’m going to partially disagree with the above poster. In terms of which is “better” for college admissions, it’s really flipping a coin - there is just no difference. Having said that, if it were me, I would take BC. APCS is fine for those students who are trying to decide if they want to take more advanced classes in college. You already know that you want to major in CS. Although I would not do anything based upon MIT’s AP policies, since admissions is a crapshoot, but MIT gives no credit for AP CS.
Calc 1 and cald 2 are pretty standardized, from college to college, and HS to HS, the AP designation, notwithstanding. CS curriculum is hugely dependent upon the individual college, so you’re better off learning the college’s way.
Again, just my opinion of what I would do. For you, choosing one option over the other is not really going to make a difference in life.