There is no "supposed" about it. We know exactly the reason for it. The story goes back to the early 1950s and the start of the Eisenhower era. (This is based on a post in the "nukes in orbit" string and is complementary to that.)

Eisenhower had a very specific strategy for dealing with the USSR. It was called containment. Essentially, it meant bottling the USSR up then hitting them with challenge after challenge until their economy collapsed and their society imploded (that's more or less what did happen by the way - later). So, the plan was to build a masisve offensive nuclear capability, when the USSR tried to match it, build a massive defense capability that neutralized that offensive capability then hit them somewhere else. The greatest strength the US had was its economy; the Eisenhower plan was to cut US defense expenditure back as far as possible and thus stimulate the greatets possible level of economic growth. For reasons that don't concern us here, armies had got to be very expensive things in the decade between 1945 and 1955. The US could achieve great things by slashing the army back - the beauty was the strategic position of the USSR meant they couldn't do the same.

The US Army was slashed back to being a tripwire. The key to the Army role was a thing called the Pentomic Division, a division structured in such a way that it could only fight using nuclear weapons. That meant if anybody attacked a Pentomic Division, it would mean an immediate nuclear reply - and the devastating American superiority in nuclear firepower made that national suicide. So puttinga cheap, light pentomic Division in the path of an attack stopped that attack dead. Either the attack ended or the war went nuclear.

The Army hated the Pentomic Division. and waged a constant war against it. One of their converts was John F Kennedy. He accused the Eisenhower administration of allowing the army to decay and ridiculed the Pentomic Divisions (in fairness JFK was probably too stupid to understand the rationale behind those divisions). JFK wanted a big army so he could go into third world confrontations with the USSR (something Eisenhower wanted to avoid - that was the job of the Marines and regional allies).

Building a big army requires a cadre of instructors and personnel. And, by the early 1960s, armies required a lot of money. Kennedy looted the only part of the Army that had prospered under Eisenhower, the Army Air Defense Command (ARADCOM) to provide that cadre. ARADCOMs budget (the ABM effort) was gutted to provide the funds for a new US Army. That was the Army that went to Vietnam. Kennedy ALWAYS intended a mass deployment to Vietnam - the NSC papers make that very clear. Johnson simply followed the Kennedy strategy. The reason why there was such a long delay before sending massed forces to Vietnam was that there were initially no massed forces to send. It took time to train the new Army, time to reorganize the Pentomic Divisions as regular line units and time to get everything in place.

If the theory of the "Vietnam" conspiracy is accepted, it has to be believed that "they" killed Kennedy for doing exactly what they wanted him to do..............