Seems like it is less about the trimester system than it is about the AP exam dates being about 5/6 of the way through the school year, so an AP course needs to cover the material faster than at a school where the school year is shifted earlier so that the entire school year is before the AP exam. Many schools on ordinary semesters have the same situation.
The other thing to consider is that AP courses are not offered, what would the offerings be for students ready to learn more advanced material? If they are other courses associated with external exams (e.g. IB), how would those exam dates fit into the school calendar?