My guess is risk is part of the answer. They don't have a huge budget, and it could be pretty financially disastrous to them if the experiment fails. And this has worked for them for 50+ years so there's no pressing need to change it.

Also, school districts are in the business of education, not transportation, so they may not see themselves as the people who should be leading in this area. If someone else (transit agency, airport shuttle company, etc.) tries something and proves that it works long term, they may then feel comfortable following along, but as far as I know, that hasn't happened yet because by definition it takes a long time to prove that.