The same reason vintage Ferraris go up in price. Are they "better" cars than new Ferraris? Objectively, no. But they are older, and that comes with it's own allure in the marketplace.
If someone wants to buy a retro "golden age" instrument from a builder, whenever that golden age may be, which is of course dependent on the opinion of the prospective buyer, then there aren't any more of them being made. The pool of instruments which may come on to the market is only getting smaller and smaller.
If Ken retired fully and had not passed his name and designs on to Kev Brubaker to keep building them, I think the prices would be even higher now. We still don't know what Ken will do after he retires, if he ever does retire that is. He has a son, so maybe he will take over the KS name and keep the basses coming through one means or another.
I agree with you. I though KS has a son, would he take over?