The US bombed North Vietnam thoroughly, but to my understanding the US never launched a major ground invasion to capture Hanoi and end the North Vietnamese leadership. It seems to me like the US simply let the North Vietnamese state operate without non-bombing based pressure while they focused all their effort in South Vietnam. Would it have not been more effective to defeat the North Vietnamese in North Vietnam, to stop the North Vietnamese from supplying the south? I'm curious why. Grasias.
In brief: because they feared that this would be seen as escalation and bring the People's Republic of China (or even the USSR) into the war more directly. If that occurred then you're talking about a war with two nuclear powers, one of which had an overwhelmingly huge conventional army. In such a situation the US either starts letting nukes fly with a country that has its own (not great) or gets into a much deadlier war (also not great). All of which adds up to a bad time.
But doesn't that mean that Vietnam was a hopeless quagmire and terrible idea from the beginning? Well, yes, indeed... that was a criticism made then, too, from both the side of those against the war, and from those who felt that if it was going to be fought at all, it ought to be fought "for real."
To be sure, there were military generals who recommended moving troops into North Vietnam, but this was consistently rejected by the Presidents involved, because of the escalation risk. Said military generals also believed that escalation was likely in such a scenario, and argued that tactical nuclear weapons should be made ready for use. The US presidents involved (Kennedy, Johnson, Nixon) rejected these suggestions.