Could that be because all they were fed were how we lost it and were losers?
Part of the reason is because the fighting in Vietnam was hard for the public to follow. We were fighting a defensive action leaving the North as a sanctuary.
America wins wars on the offensive, conquering and overrunning enemy territory. We haven’t had as much success in defending against drawn out insurgencies. Neither Johnson nor Nixon were willing to invade the North and capture Hanoi.