The American hi-tech tactics continually killed the wrong people and demoralised their own troops. The Vietcong's guerrilla tactics were appropriate to the nature of the conflict.
The US was trying to supply a war 8,000 miles from America. The Vietcong were supplied with weapons by China and Russia.
The South Vietnamese regime was weak, brutal and corrupt. The South Vietnamese peasants supported and sheltered the Vietcong.
Their short (one-year) tour of service meant that American troops were always inexperienced. The Vietcong had been continuously at war since they resisted the Japanese during the Second World War.
The morale of Americans soldiers was rock bottom - they took drugs, shot their officers ('fragging') and deserted. The Vietcong were fanatically determined to drive out the Americans, whatever the cost.
The war became very unpopular in the US, and lost public support. The North Vietnamese were motivated, fighting at home to unite their country.