In short, the answer to this question is that bombing the dikes would have been a bridge too far for Lyndon Johnson and his advisers, both civilian and military. The Red River delta, in which Hanoi is located, is the most populous region of Northern Vietnam. The dikes are necessary to control flooding. Had the US destroyed the dikes, the resulting floods would almost certainly have devastated North Vietnam and killed hundreds of thousands of civilians. You might be asking yourself why that would have been a problem, given that the Allies used air power to similar ends during World War II against both Germany and Japan. The answer is that Vietnam, from the beginning, was a fundamentally different kind of war than WWII. When Johnson chose to escalate the war in 1964/65, he did so with the intention of keeping the war limited. Part of that meant trying to spare the North Vietnamese civilian population from the worst of the bombing. Doing otherwise would have meant risking international censure, to say nothing of the negative domestic reaction. Hanoi took advantage of this policy by placing anti-air and SAM emplacements near or on top of dikes, which left them off-limits to US strikes. Rather than the civilian population, US bombing raids, particularly during Operation Rolling Thunder, targeted North Vietnam's infrastructure and industrial capacity. This refusal to bomb the dikes continued in Operations Linebacker and Linebacker II.