Why do you think Operation Rolling Thunder failed to lead to a quick victory?

It assumed that it would destroy both the infrastructure and morale of North Vietnam and its people. It failed on both counts.
Operation Rolling Thunder was one of many examples where it was assumed that the massive use of military might, in this case bombing would lead to a quick victory. Instead it had the opposite effect.
An earlier example wold be the Blitz of London and other British cities during World War 2. Despite Goering’s grandiose claims it failed to break the spirit of the British and in fact strengthened their resolve against the Nazis.
Similarly through operations such as Rolling Thunder, the Americans only strengthened the will and the resolve of the North Vietnamese in their fight against what they saw as American imperialism. This in turn meant that they could keep the war going at a level which drained American resources and political will.
The Vietnamese war was not just a guerrilla war. The NVA had significant numbers in the South. However much of it was a guerrilla conflict and in such circumstances the guerrilla do not have to win, they only need to sustain the conflict.
Operation Rolling Thunder was another example of American arrogance in assuming that they would secure victory through sheer military superiority. It was they who talked about Hearts and Minds, but it was a concept they never successfully understood or implemented.