At least three reasons in the 60s:
1. Noise abatement - the locals like a break from the noise and the noise lobby is unbelievably powerful.
2. Traffic rarely offered equally from/to both north and south, resulting in frequent ground control and radar problems.
3. Heathrow is a very small airport and ground control with both runways being used for landing and take-off proved extremely difficult.
There are probably other reasons too.. Have you seen how quickly weather changes in the UK?? It would be almost impossible to run a sequence based on good weather more than 2-3 days a year!