Large main battle tanks for instance are not needed as much in jungle warfare. They are needed for open field combat like the kind that would occur on the fields of Western Europe. So those types of units were largely sent to Germany, Korea, and kept in the United States.
Aviation, infantry, artillery, etc., are needed in places like Vietnam. Or are you going to tell that veteran with a "Big Red One" or "Screaming Eagles" patch on his shoulder that he was low quality?