So that all the massive military effort at great cost of lives and money would end with the Communists obtaining their goal. Not that I necessarily think the US should have been involved, but the political power of the Communists proved to be more decisive than any military effort. With the MSM acting as its ally.
Vietnam 1965: The Day It Became the Longest War
http://extendedremarks.blogspot.com/2006/12/vietnam-1965-day-it-became-longest-war.html
“The bold measures they would propose were to apply massive air power to the head of the enemy, Hanoi, and to close North Vietnam's harbors by mining them. The situation was not a simple one, and for several reasons. The most important reason was that North Vietnam's neighbor to the north was communist China.”