I thought that was due to phones being relatively low power and all the central offices having battery backup.
That was also part of the reason. The power source was independent of the power grid in event of emergency. The older system had huge batteries and a generator generators depending on the office to charge them in event of power failures.
When MA Bell began converting over to electronic switching in the 1970's this also cut down on the batteries needed. Before then the switching office for even a long distance office in place like Ashville, Nashville, Knoxville, etc was about as large as a Walmart. That was just for the long distance hub for a given region in a state. The replacement was electronic which it and a back up system would fit in an average size living room.