Why were the Vietcong so successful against the United States?

Because the Vietcong were fighting on their home terrain and also were able to employ very sophisticated guerilla tactics which negatively affected the morale and desire of American soldiers to even fight there in the first place.

Two of the major reasons why the Vietcong was so successful against the United States was because they practiced "guerrilla warfare," which allowed them to constantly ambush US forces--and because they were being supplied with weapons from China and Russia.