Because in our country public transportation sucks.
It would be cost prohibitive for private companies to maintain a fleet a busses for use twice a day nine months of the year so each school district comes up with their own plan for transportation.
I walked 8 years to Catholic grade school and rode a yellow bus to Catholic High School until I got my own car. The bus was awful, no supervision so a lot of bullying went on to and from school.
Our High School served part of Detroit, those kids got bus passes to ride city buses to school. 40 years ago the city bus system was fairly reliable, today, not so much.
I think yellow school buses have to do with safety.
