The old up Catford Loop platform was very tight up in the junction V with the Chatham Main Line. That would mean under modern rules that in order to draw up into the platform a signal overlap would have to extend beyond the platform signal right through the junction, locking out all other simultaneous moves on the main line in both directions until the train had come to a stand. If any Main Line movements were taking place already then the Catford Loop train would have to wait until they'd cleared the junction before it could be signalled in. You also wouldn't be able to make simultaneous arrival moves into both up platforms as their overlaps would interfere. Mechanical interlocking had similar controls typically, so I expect it proved operationally impractical to call there routinely as it would overly constrain capacity on the main line, and generate unpredictable disruption in the case of even minor late running. I think there's a restricted overlap there today with the signal set back a little from the junction. That means if the junction is blocked on approach you get checked for a warning route at the previous signal. A new platform might be built further back from the junction with the signal moved to get a longer reduced length full overlap suitable for the approach speed to tackle the problem but that means longer passageways, crossing more roads to the east etc so probably adds to difficulty and cost. A new down platform could be built closer to the junction as there's no overlap issue.