When concrete slabs are poured what happens? They cut lines in them so when they crack, it gives them a crack point and keeps cracks down. Not IF they crack, but when. So your pool bottom, if done in concrete, would need lines cut in it which would not look good under a liner. In addition, vermiculite is porous which allows water to pass through. If you had a ground water problem, and you used concrete, I suspect it would just start cracking the pool floor to pieces.
As for poolcrete versus vermiculite, since poolcrete is basically portland/vermiculite already mixed, it is simply a labor issue. Rather than lugging 90lb bags of portland and mixing with vermiculite, you simply dump a bag of poolcrete in and mix away.