The M60 was introduced in the early 60's, well before the major American buildup in Vietnam. Yet, instead of M60 tanks, inferior M48s were sent instead. Was the U.S. just sending its older second-stringer tanks to Vietnam and saving the M60's for a possible brawl in Europe with the Soviets and Warsaw Pact?
There are a few different reasons the first being that it wasn't really necessary to bring such high tech weaponry when facing a relatively less technologically developed opponent, its not like in ww2 when we were facing the german super tanks. The next reason is that the US didn't want to risk their top of the line technology falling into enemy and eventually into Soviet hands. The final reason is the whole possibility of WW3 and wanting to protect the homefront.