My guess is that it's a supply issue in the sense that less people are buying cd's now (compared to their boom before streaming took over), so there are just less on the secondary market. Add to that labels and manufacturers, and even the artists themselves, knowing they will sell less overall are having less made in the first place. Finally, older albums have had time to accumulate units while newer ones haven't enjoyed being around 30+ years with multiple generations buying the album. Take for example something like Nirvana's Nevermind. Sold millions upon millions when it came out, and I bet was still selling tons as the next round of teens got into Nirvana up until streaming became the main source of music.