A related question, and may I say, a very interesting one. The "UNC" percentage seems to vary considerably from coin type to coin type, and although there is obviously some absolute relation to overall rarity, there are some definite real terms differences, which can be difficult to explain. For example, off the top of my head, amongst the rarest coins there are some, such as the F14 penny, the F8 and F9 mule (all 1860), and the 1862 F38 mule, which seem to have more than you would expect as UNC examples. There are others. Whereas such pennies as the 1864 (plain and crosslet), and the F98 1879 narrow date, which are not that desperately rare in lower grades, are extremely difficult to locate in top grade. The F90 1877 narrow date penny, which is roughly on a rarity par with the F8 & F38 mules, has nothing above fine among its ranks.
Obviously I'm talking exclusively pennies here, as it's my specialism, but the same principles must apply across the board.
In some cases I think there are more UNC specimens available because they were collected by default shortly after mintage, as part of a date run (the collector having no idea of the coin's significance at that time). In other cases, it's very difficult to know the precise reasons for either a surplus or deficit of UNC examples. I imagine we never will and are left to speculate.