Taking a guess here. Maybe in the past, when metro had more staff, the turnaround would also be the point where the train operator changes, giving the incoming one a break. With two operators, you could effectively minimize the time the train sits out of service as control changes from one end to the other.

But now with the staffing shortage, you're more likely to see the same operator operate the same train on the way back, meaning there's a more noticeable amount of time the train is "out of service" while the operator traverses the length of the train.