There's always more to be said, but you may find some answers in this thread.

This was the era of the Cold War. The US administration was wary of taking any actions that might provoke China or Russia into getting involved in the war. It was decided that a large scale aerial bombing campaign might be the best way to fight North Vietnam without committing ground troops.

Both of these strategic assumptions proved to be mistaken, as China and Russia continued to supply North Vietnam with arms, aircraft and war materiel. By 1968, America had committed half a million troops to the conflict and the bombing campaign, Operation Rolling Thunder, had failed to disrupt NVA and Viet Cong operations.