Why do people call a cup of coffee a "cup of joe"?

Unfortunately, there is no way to answer this question with ia "the slang term joe for coffee came from..." I can, however relate that the first answer is highly unlikely. The most plausible (and therefore likely) anwer according to 2 etymologists on the web I consult is that "Joe" is related to either the use of Joe as the common man (coffee is the common man's drink) or as a derivative of Java. Boring I know!

The previous response referenced a story that I've also heard used to explain a "cup of joe." However, while it is a great story, there is no support for it.
I go to 2 etyomology (study of word origin, though the literal breakdown is "study of what is real") sites on the web when I want to know about word/phrase origins. Both of them are written by professionals who specialize in etymology.

WorldWideWords (http://www.worldwidewords.org/qa/qa-joe1.htm ) indicates that the banning of the wine mess theory is unlikely because, " Cup of joe appears in the written record in 1930 but the order to ban alcohol—General Order 99—was issued on 1 June 1914." Another point the author makes is that wine mess halls were only for officers, not the common sailor,, so why would the common person be bothered enough to start a slang term.? This slang was (and is) used by everyone, not just a special group of people (the officers).

The Word Detective ( http://www.word-detective.com/041899.html#joe) is pretty much in agreement. WorldWideWords agrees with the Word Detective, who suggests that "joe" as slang for coffee might be derived from "Joe" as a synonym for "the common man," as in "regular joe." They do differ on whether
"GI Joe" for enlisted men was in use at that ttime. Even if it wasn't, US citizens jhad been using "the common Joe" and other such phrases for a whle when the ft instance of joe for coffee is written.

Of additional interest; WWW writes: It is significant that an early example appears in 1931 in the Reserve Officer’s Manual by a man named Erdman: “Jamoke, Java, Joe. Coffee. Derived from the words Java and Mocha, where originally the best coffee came from”.