Gary Gygax created AD&D's planar arrangement, although it was diagrammed as a square back then. I think that it first appeared in the 1E PHB (it may have been in a Dragon article prior to that).
There were 16 outer planes in the original (Concordant Opposition didn't appear until Deities and Demigods, which is also the first place the outer planes were diagrammed as a wheel). The 16 planes come from 8 planes corresponding to 8 of the 9 alignments and 8 "buffer planes" between them, representing shades of alignment. So the Twin Paradies lies between the Seven Heavens (Lawful Good) and Elysium (Neutral Good) and are the afterlife destination for those who are very good, but only a bit lawful.
I don't know why was there no neutral alignment plane. Maybe it was assumed that neutral souls simply reincarnated until they picked a "side"? In original 1E, there were no true neutral clerics either (or rather, the only true neutral cleric-types were druids, devoted to nature) so there's a certain consistency there.