There were a lot of real culprits. The South's government was corrupt; our government was never really serious about winning - only about not losing; our tactics were frequently faddish; an emphasis on military management rather than military leadership alienated the troops; bright pronouncements of impending victory which went on for years led to disillusion at home... and it has to be said, the NVA was stubborn enough and good enough to take advantage of our shortcomings.
If we could have won the war we would have. But wars are between cultures and at the time our culture just wasn't good enough.
And every time we were close to winning, we stopped the B52s.