I'd also have to add the overwhelming infusion of cash into the America economy from the arms industries and the production of war material for World War I. $17 billion worth of arms sales to European empires took place at that time, and this created not only massive wealth in the hands of a few industrialists, (which they were able to invest later in new economic ventures) but drove up wages, caused increased consumer spending, and increased stock prices. The economy expanded greatly as a result, as did its industrial base.
Republican laissez-faire administrations over the course of that decade that kept taxes and regulations low sure didn't hurt, but I believe they merely accelerated an economic trend already brought about by the vast fortunes of war, and a new American middle class who wanted to share somewhat in the luxury of the new economy.
From the first link below:
Annual incomes rose steadily, from $580 in 1914 to more than $1,300 by the end of the decade. Thus, World War I dramatically changed the domestic and international economy and set the stage for the prosperity of the 1920s.