What were the reasons behind Americas failure to achieve its objectives in the Vietnam conflict?

The spread of communism wasn't checked; the communists won. The war was un-winnable because the battlefield (South Vietnam itself) couldn't be isolated. Korea had been a peninsula and was easily isolated because the USN controlled the seas. Vietnam was not a peninsula. The war either had to be expanded into Cambodia and Laos, and risk war with the USSR or Red China (as had occurred in Korea) or abandon the war altogether. The US chose the latter.