Like I said, they've become obsolescent with the advent of modern popular accessories. Back in the day the SKS was more of a niche item, a curiosity, a relic that no one considered much more than a big bore plinker. Now folks use them for three gun, hunting, defense, action shoots, milsurp shoots. Its popularity has skyrocketed thanks to new supply and availability of cheap ammo, and accessories to bring it into the modern age that make it competitive with other designs.
Why bother spending $400-500 on a used SKS-D that takes hard to find and expensive AK mags (Marstar for example has them at a whopping $75/piece) when that same money will get you a beautiful Tula refurb, four Tapco 20-rounders that DO hold the bolt open and charge with strippers, and a full case of ammo?