One American coin series is special because of its specialness. Every coin in this series is a rarity. Only the very rich and very serious numismatists would even attempt to collect this set.
The $3 gold piece was struck from 1854-1889. In the era of common coins in uncommon condition selling for big prices, the $3 gold coin is genuinely rare, simply because not many were minted. The coin with the highest mintage was the first year 1854, with over 138,000 struck. The next highest is the 1878, with only 82,304 minted. Compare those figures to the modern coins, or even to some more famous rarities.
And when a collector desires proof $3 gold...that's dealing in a rarefied atmosphere, to say the least. Proof mintages can be as low as 25 or 30 minted. How many survive?
If you are attending the ANA special day at the WFM, pay a visit to the Harry Bass Collection, on display at the ANA Museum. The unique 1870-S $3 gold is there. I made a point of admiring this wonderful collection, and taking a long look at that unique coin. Look at that coin, admire it, appreciate it. It's a unique coin in a one-of-a-kind coin series.