Answer:
the 1st and foremost reason was that if USA openly attacked the north Vietnam, the secret would be a secret no more and the whole world will realize about the plan of USA to curb the spread of communism even through military interventions!
secondly, there was the risk of Chinese intervention if USA engaged in direct conflict with North Vietnam. and this was a scenario that many US Presidents was unwilling to take as it could have had many negative reaction from the international towards USA.
Explanation:
