I believe it would be the same reason why America didn't nuke China during the Korean war. Due to the fact that it might've started a WW3/
Douglas MacArthur wanted to drop 8 atomics bombs at China, But it was the fear of the Soviet Union getting involved.
nuclear weapons will set off world war 3
Because it's barbaric
The Vietcongs have not done anything harmful directly to our women and children. It's why most Americans wanted to pull out of Vietnam.