Two big things. First, because the economics of modern pinball are brutal, venues mostly stopped buying, so manufacturers and their competition shrivelled up and with it, supply. The supply-side market at this point is effectively dominated by individuals and collectors, neither who want to part with their machines, as they’re nostalgic items and they know they have value.

Furthermore, working machines come at big premiums as repairs are finicky, expensive, and time consuming - god help you if you own a machine and some targets or rollovers on your machine give out, as it’s challenging to find someone who’s even capable of doing the repairs, let alone pay their price. The expertise to do it is really thin these days because all the people who historically used to make a living from it are all getting old and retiring. This combines with a second problem: although widespread 3D printing helped a little with making repairs more doable, parts are hard to come by and often demand cannibalizing other old or dead machines - meaning a working machine is worth its weight in gold, and so is a dead one: Either it has interested players, or it’s of value for salvage. This “literally every machine is valuable” situation drives prices up, even if the machine is essentially dead.