I wasn't aware that Back In Black had sold that many copies, I knew it was high up the list, but that figure kind of surprised me.
Most of the albums on that list were kind of "perfect storm" type albums, they were released at just the right time and managed to become both huge hits and also highly influential on everything that came afterward. So they have staying power.
The Santana album only really applies to the "hit equation" of that paradigm.