"contemporaneous"
Thanks Marc, word of the day for me.
As mentioned above, the word "rare" gets discussed here often. Supply & Demand dictate "value", but the terms [rare, scarce & unique] relate specifically to the number of items still in existence. They are separate and distinct from "demand". Put the two together, then there is your market value.
Getting even more specific to your question- a lot of "rare" sets, brands, cards, etc. were regional, only distributed in certain areas of the country (southern league T206 cards for instance). So the supply is lower on them compared to cards with a larger distribution, hence the shortage of examples.