I also noticed that more popular, older titles don't come up for sale as often. This probably has 2 main reasons:
1. Popular games tend to have longer hold times by their owners while boring games are sold more frequently.
2. Popular games where played to death when they were new back in the 70s and 80s. Then parted out and junked.
For example Joker Poker (1978) had 10K games made but it is difficult to find these days. Given the electrical problems many early SS Gottliebs had they probably didn't stay around very long and were dead after a few years.
Seawitch (1980) had 2503 games made and is extremely hard to find.
The popular TOTAN, AFM, MB, ... had 3+K games made and comes up for sale very frequently. By the mid 90s games were better quality and weren't played as much anymore. That's why, based on production numbers, relatively many 90s titles are still around.