It's because an attack in the North would almost certainly have forced China to take action, as it had done in the Korean War. American troops in the North would be directly on China's borders - which would probably result in direct confrontation and absolutely bring a risk nuclear war. Anyone who grumbles about America's "hands being tied behind its back" is forgetting this key point. The American wartime objective was to maintain an anti-communist government in South Vietnam, not destroy the government of the North.

You might be interested in some histories that would help clarify things a bit better. I recommend the late John F. Guilmartin's America in Vietnam: The Fifteen Year War, and Lien-Hang T. Nguyen's Hanoi's War: An International History of the War for Peace in Vietnam.

As has been described already in this topic, the primary fear was triggering a Chinese intervention in South Vietnam. In Strategies of Containment: A Critical Appraisal of American National Security Policy during the Cold War, Gaddis describes the way in which the administrations of the time attempted to balance the desire to bring as much force to bear as possible without crossing the threshold at which the war might involve the Chinese directly.

As described by Gaddis, during the initial phase of the war, most of the manpower was coming from South Vietnam, while North Vietnam supplied support, training and equipment. Later on, though, the manpower burden shifted increasingly to the North Vietnamese as their large offensives failed and new recruits were not able to be found in the South at sufficient rates to cover their losses.

The Nixon Administration took several risks that the preceding administration did not - extending the war to Cambodia and Laos, and the mining of North Vietnamese harbors (Operation Pocket Change). The administration's primary interest though was simply to bring North Vietnam to the negotiating table, not to "defeat" it in a conventional military sense of taking control of the North to dictate terms.