1) Americans just in general tend to drive their kids to school dues to fear of child abductions and related ever-present paranoia.
2) Boston's transportation system is broken on just about every levels. The street organization is irrational, derived haphazardly from ad-hoc pre-auto paths. The public transit system was also built in an ad-hoc and is at level where full transit lines stop for days at a time. So all that's left is driving, though Boston drivers have a terrible reputation too!
3) Boston's weather is also miserable - the place is at just latitude so that wet, 30 degree weather prevails in winter, giving freezing rain, frozen rain, slush and all manner of nastiness. Plus horrible hot, humid summers. So you have to avoid walking a lot.
4) Boston experienced court-order busing to end segregation in the 1970s, so it's often true that students aren't in schools closest to them geographically.