Scarcity, Rarity in a Demand Driven market.
The truth is that all markets are essentially demand driven.
If everyone suddenly realized their I phone 37 was essentially exactly the same as their I phone 3 Apple profits would disappear.
If everyone suddenly realized commodities-trader turned "artist" Jeff Koons was essentially making fun of his own clients a Balloon Dog would suddenly have no value at all.
(rather than 55 million dollars.)
Demand for drugs is why no policy directed at curbing the supply of drugs is ever effective.
Demand for capital in a debt ridden economy is why Supply Side Economics is suddenly impotent.
The coin market is another Demand Driven market. Take the "rare and valuable Mint State flowing hair large cent. "Only" 1200 1793 lots have been auctioned with the high hammer having gone at close to a million dollars.
Now, in other areas of the coin market, say Ancients, or Medieval coinage, if any particular piece had come up for auction 1000 times, it would be considered the most common coin available, and even coins of excessive beauty, and historical interest would have trouble breaking the $5,000 dollar mark were they that common.
For example, a particularly nice mint state Daric of exceptional style, might have come up for auction 10 to 20 times, while run of the mill mint state daric probably has come up for auction 100-200 times (85 have been graded mint state or better - which seems like a tremendous amount in the current ancient coin market) Still, a Gem daric only hammered at $25,000 and nicest fines style mints state Darics only make slightly more that $15,000
This is where supply comes in. In very tight markets small shifts in demand can cause large price bumps, and pieces that once seemed available can disappear off the market
The most difficult thing for a collector coming from US coins to get their head around, is how relatively tiny (compared to the US markets) these markets are - in many areas.
For example, about five years ago a horde of 20 shooting darics hit the ancient coin market.
MS graded examples sold for about 20,000 dollars. For a year or two every major auction seemed to have one of these. High end collectors knew they could pick one up if they really wanted one. At that time 20 coins could have that effect on the Ancient Coin market. These are gone. It's been over 2 years since one in any grade has been auctioned. More may be found. Or maybe not.
I use Darics as examples because these were from 500 BCE to about 336 BCE the reserve currency of the Eastern Ancient world - while Athenian Owls and Kyzykian Staters were the reserve currency of the Western Ancient world.
As such, the Darics are easily the most common Ancient Gold pieces along with the Alexandrine Gold Staters that replaced them after Alexander the Great.
But the fact is there are many more MS 1793 Flowing Hair Dollars floating around than there are MS Darics.
There was a relatively large and recent horde of Darics which has still not been fully absorbed by the Ancient Coin Market. But for new collectors, wait about 2-3 years and as the market grows and the horde is absorbed, my guess is that mint state Darics will become quite difficult to find at auction or in dealer inventories.
You can extend that logic to other areas of the Ancient Coin market that aren't nearly as plentiful.