Because state schools subsidize their in-state students (which is most of their students), they have a financial incentive to get students graduated as quickly as possible. On the other hand, private schools may have a business model which depends on getting enough net tuition out of each student, so a student who graduates early hurts the school’s finances.
However, the above mostly applies to trends regarding credit units for AP scores. Subject credit and advanced placement have other considerations that matter, such as whether the departments observe that students with a given AP score in a given AP test do well in the subsequent course after skipping the introductory course with the AP score.