Many reasons

In reviewing Dutch's Journal entry, there are several reasons (some which make sense and others not so much) why a coin with a 3M mintage is more valuable then one with only 300K. For Morgan dollars, this phenomenon is usually the result of which coins were placed into circulation and when and which issues were more prone to melting in the "silver rush" of the late 1970s. Reading Q. David Bowers' Encyclopedia of US Coins is a good resource for finding out all of the "condition rarities" for Morgan dollars.

The 1884-S is an interesting choice of Dutch's ire. While it does have a large mintage and is plentiful in circulated grades, uncirculated varieties are hard to come by without leaving a large hole in the wallet. Just as interesting is the 1844-CC which has a much lower mintage but can be had in uncirculated in some cases less than $200, where as for the aforementioned 1844-S, MS-60 and up can be in the multiple thousands.