People who don't own a copy are likely willing to pay stronger for an entry level copy, or one with minimal eye appeal.
People who already own copies in higher grade likely do not participate in bidding anymore, either they are happy with what they have or cannot afford the higher grades or don't want to.
And as @lou_fine mentioned, there are multiple copies at every single auction, all the time. At one point, the buying pool thins
out and you are left mostly with dealers re-building stocks or the occasional attempt at an upgrade.