Analog was Faster
This may surprise you, but analog central offices could from the outset and up to their demise transmit data faster than digital ones.
There is a reason that the original picture phone service only worked in analog central offices, and that was because there was 1~5 MHz of bandwidth through the analog switching fabric and only 64 kilobits through digital switching fabric.
It was long possible to build modems that would transmit an order of magnitude faster data through analog central offices than digital ones. I saw a design offering the unheard of 1-megabit speed city-wide as long as the connections were analog. This was 10 times faster than digital would ever do.
But the reason that high speed analog data was never rolled out is the same reason that picture phone service was never rolled out. It was based on analog technology and the technology roadmap for long haul circuits was digital. Satellites were increasingly used in the network and they were often digital. The new monster-sized #4 ESS was to be all digital and this switch was destined to be the backbone of the nationwide network.
Data compression was too expensive and required too much processing power, so for two decades the speed of digital switched lines remained a pathetic 64 kilobits.