It's a good question but say they want to bring power back up to the neighbourhood as quickly as possible, and they would have to go to each house and shut off each connection which would take a while even if it were accessible outside without needing to inform the property owners. One residential transformer usually serves from 1 to 15 or so households. Many transformers are on one branch that extends from a substation. So depending on where they isolate, just one customer out of hundreds can backfeed which can make the entire branch dangerous to work, since normally it is all expected to be off on the other side. Putting the responsibility on the utility to isolate every downstream point is time consuming and just extends blackouts. However if they were to isolate smaller sections up and downstream it might make it easier.