LBJ and McNamara didn't want to take the political hit of calling up Reserves or Guard units while most of the active forces were already busy with the Cold War. But there was the Draft
New Draftees got churned thru S.Vietnam, with half of their Tour in Country, and they back to the States for the remainder, where they did very little except mark time, Drills that sort of thing.
So by time the troops were no longer green, and knew what they were doing, were rotated out. This meant when the FNGs got dropped into an existing unit, they would be a hindrance to the rest of the guys thill they picked up skills, at which point, they would start feeling 'Short' with the end of their Tour in sight.
So obviously no wanting to get send home in a Bag or or shot up, would start being very risk-adverse. So a Draftee might be only combat effective for 4-5 months in Vietnam.
You may ask, 'what about the guys who volunteered'? Well, those guys had a longer tour, but in most cases, they would have a different MOS, and a choice of where to go, within limits.
Only the most Gung-Ho and crazy chose Vietnam.
Draftees, most ended up as MOS 11B, Rifleman, unless they tested well on the AFQT (Armed Forces Qualification Test) where they might go to another Branch and/or MOS
The lucky guys who had taken typing in high school, were very likely to be MOS 71B, Clerk-Typist, who would spend his tour in an air conditioned MACV office than an 11 Bulletstopper.
Or if you scored well on Electronics or knew anything about Ham Radio, you might get take care of Radars or similar in West Germany, South Korea, or the States, especially if you went for a regular 4 year tour