Guerrilla warfare was so successful in Vietnam because it was a jungle climate that US soldiers were not used to. So it made it a lot more difficult to fight in an unfamiliar environment especially with that warfare.
Guerrilla warfare is basically 'Hit and Run' with a more sophisticated name. I'm not sure about why it was so successful in Vietnam, but I do know that Guerrilla warfare is very effective when executed properly, because there are very few casualties and a lot of damage is done to the enemy. It also works really well when the people using guerrilla warfare know the terrain well, because they would have an advantage over the enemy.
