they were never advertised, leading to low demand, so fewer were sold? if so, them being uncommon could lead to the old supply/demand thing.
it could also be the same thing (which i don't understand) that causes prices of extremely popular games to sell for high prices. Look at pokemon or zelda carts, for example. These were really popular and sold well, and they are very easy to find now. For some reason they regularly sell for higher prices than less common games, even though they are plentiful. I suspect this was caused by some people who were willing to pay more for their favorite games, then people saw the price paid and asked for or paid more thinking it was what they were worth, and now those higher prices are what they are worth because of some weird marketplace feedback loop. If anyone has a real explanation for this phenomenon please share it, as i find it very interesting (and annoying, as a buyer).