Because the 63 goes to Elephant and Castle which is a much more direct route than trying to get on a P4 or P12 when you want to try to reach the other side of the river.

During the snow for example when the trains to London Bridge and the Overground were not working, my easier alternative was to get on a 172 or 171 to try to reach zone 1. The 63 would have cut the detour to New Cross and probably saved 25/30min of that Journey.