So that it took longer than a year to send combat troops to largest war in history at that point, and against the strongest military in that war. Why did the military was not battle ready prior to 1914, and why nothing have done to build it up between 1914-1917 while war was raging in Europe like hell and was at America's door. When US troops started to deploy to Western front in 1918, most used British/French artillery, aircraft, tanks and other many equipment. This is interesting considering that this same military was to become very well organized, most technologically advanced and biggest in the world less than 30 years after that.
America had back then (and to an extent still does) a deep isolationist streak. One of our Founding Fathers (some sources say George Washington, some say James Madison) said standing armies are dangerous to republican liberty so having something like the massive German and French armies was never in the cards for America, politically speaking.
Then there's the fact that standing armies are ridiculously expensive to maintain, and IIRC the US at that point was more interested in investing in the navy.
As for the industrial power of the US in WW2, I think that was largely a byproduct of coming out of the Great Depression. Recall that the US military was actually grossly unprepared then too. I believe they had the 17th largest army in the world behind Romania at the outset of WW2.