While annoying myself researching the disgraceful histories of US presidents who carefully avoided military service during their Vietnam War era by various forms of undue political and other influence and base dishonesty (Clinton, Bush the Younger, and Trump with his laughable temporary bony heel spur, which I had and cured by simple exercises as a bony heel spur is bullshit) but who were belligerent in office in happily sending to war exactly the same sort of less privileged average Americans who did their national and military duty when these over-privileged bastards shirked theirs, I noticed this on Clinton:

As Clinton headed home for Arkansas from England, his options for avoiding the draft were limited. He did not qualify for conscientious objector status because he did not have a history of opposing military service or war in general, only the Vietnam War specifically. The local Army National Guard and Reserve units were full.
https://www.snopes.com/politics/clintons/felon.asp

I understand the distinction between US National Guard and Army Reserve so far as federal and state government control is concerned, but I don't understand how both National Guard and Reserve units could be full so as to exclude Clinton, unless they are organised purely on a local geographical basis so that Clinton couldn't join.

Were those units full at the time because people joined them to avoid service in Vietnam?

If so, how does that make sense when both National Guard and Army Reserve Units were deployed in Vietnam?

Was the attraction of National Guard / Reserve (Young Bush was Air Force Reserve) for future presidents simply that there was a very much lower chance of deployment to Vietnam in those forces than as a draftee?