I could only speculate about "the" reason, but I think that there were enough ideas floated that people latched onto one, or more. The cold war was in full bloom and the military thinkers didn't want Russia to have access to a year-round fair weather navel port on the coast of Vietnam. America was not a pre-WW2 colonialist in Southeast Asea, that was America's friends, the French and British. We had no historical attachment to the region or the fate of its inhabitants.

It is easy to get people to go fight in them. I came from a long line of military veterans on my mother's side of the family going back to a roster of privates in the Revolutionary War. There was a war going on and young men like me didn't want to miss it. Interestingly I am the last in that long line, there are none in my extended family who followed suit.

If there is a root cause reason, Old Gimlet Eye probably put his finger on it. https://papersourceonline.com/war-is-just-a-racket/ In the longer version of this he described war as something where profits are in dollars and losses are in lives.