A few thoughts
Politically it became very difficult to sustain casualties and maintain an army in a country with no direct threat to the US itself. In previous centuries empire's didn't have TV camera's peering at their activities and bringing the realities of war into civilians living rooms. Neither was the vote so universal.
The terrain made it very difficult to utilise any technological advantage particularly at night.
Modern 20th century armies actually used very few combat troops I recall only 50 000 US infantry were available to cover the whole of South Vietnam, the rest were support in one form or another.
The South Vietnamese became dependant upon technology to fight whereas the NVA adapted to fighting it, when the US left the communists were battle hardened, ostensibly 'trained' by the US.
The US were effectively fighting the Soviets and China as well, not just the NVA and Viet-Cong, the latter two simply supplied the troops. Worse still the US were politically restricted were and when they could strike.
The more the US bombed NV the more the population supported their nationalist cause.
The battle for heart and minds was poorly thought out. The South Vietnamese could afford to live in luxury for a fraction of US spending on the war itself, yet the villagers still lived in poverty and were terrorised by the VC and sometimes by the South Vietnamese and US as well.