With a combination of combined conditions, such as high altitude airports, of South America and the western part of the US, high temperatures, and heavy take off gross weights can cause the aircraft to approach its tire limit speeds. Not all aircraft back then were equipped with the 225mph tires. Its far safer to allow the wheels to slow their spinning effect prior to retracting the gear. This is also true if there is a wheel slightly out of balance. This also allows the wheels to cool down too. Some aircraft have inportant components in the main wheel well that could be damaged in the event of a tire burst that could effect the safety of flight if a tire were to burst after gear retraction.
This procedure is briefed before blocking out. It was understood that the gear would be retracted if performance requirements dictated.