The longer an item has been out of production, the lower its circulation. That’s not specific to Krells. But big Krells have the added disadvantage that they’re heavy as hell, hard to even move a few inches, and extremely costly to ship. Plus they may need service. These serve as a barriers to re-entering the used market. Risk is high (for both parties), and reward is much lower (depreciation).
That said, I’d like to think the remaining good units have mostly found long-term homes with owners that are really enjoying them!