The branch was added to the Circle Line to double the frequency to Hammersmith. There wasn't, and isn't, any spare capacity between Baker Street and Aldgate so more trains had to terminate at Edgware Road. They could have run the extra Hammersmith trains as a shuttle, and kept the Circle, but a circular service is always inconvenient operationally (no termini where some slack can be added to the timetable to allow for late running, or where a train with a problem, or with no crew available, can be parked out of the way). And passengers from the western side of the (original) Circle not wanting to change at Edgware Road have alternatives to go east, notably the Central Line from Queensway (very close to Bayswater) or Notting Hill Gate.
...which is why those will be the last five sections to be converted. It is even possible that the Piccadilly Line will have its new trains by then, which could simplify sections 11 and 14 if the trains are compatible with the new system.