In our neck of the woods, it's lack of money. Routes are consolidated to the extent that kids may spend up to an hour and a half on the bus! We live in a rural suburb of Austin and my kids were lucky to be at the front end of their route so they could get home in about 40 minutes (ten minutes by car) but getting them up that early was a pain so we took them, especially after I started teaching in the district. No way could they have ridden bikes -- the only route into town is a four-lane U.S. highway with speed limits up to 70 mph and no shoulders.