Doc Brown's Delorean could only travel through time from the specific place the Delorean happened to be at the time. In the first movie, he claimed that one could actually witness the signing of the "Declaration of Independence" but this would only be possible if the machine were actually in Philadelphia once it jumped through time.

Because the Delorean was always happened to be in Hill Valley it would only jump through time to Hill Valley. If I were buying a time machine, that would sound like a raw a** deal.