External Tanks
Word at the time was that it was to reduce fuel consumption when bashing the circuit, but it was realised that the performance of the aircraft was so much better, that it could mislead new crews so that when they then flew with tanks attached the drop in performance could have lead to some suprises for the crews. Also the reduction in fuel consumption didn't outway the complications caused by effectively having two fleets, one long range and one short to medium.