American involvement in WWI simply wasn't as great as in WWII. In WWI, the United States officially entered the war in February 1917, but US troops really weren't involved in fighting until July, 1918. c. 110000 Americans died before November 11th, 1918.

In WWII, the United States entered the war in 1941, and was a major member of the Alliance alongside Britain and the USSR right up to the end in 1945. American combat troops were the largest force engaging the Germans in Western Europe from 1943 to 45, while virtually the entire Pacific Theatre of Operations was overwhelmingly dominated by American forces. The scale of American involvement, the extent to which the population was mobilized, and the way that television, cinema and radios brought the experience of the war to the mass of the population, also meant that it was a more universal experience.

When WWI ended, America slipped back into isolationism, and largely concluded that they had been led on a fools errand, in which over 100 000 Americans had died for nothing (I would disagree). After WWII, the United States found itself the World's premier superpower, soon to be locked in a Cold War with the USSR; (most) Americans enjoyed greatly improved lives after WWII, and the crimes of Germany and Japan made it easy to look upon WWII as 'The Good War'.

Actually, according to Kurt Pielher, director of the Institute on World War II and the Human Experience and author of Remembering War the American Way, WWI is the second-most memorialized war in the US behind the Civil War. He also wrote a recent article on The Daily Beast and was interviewed on NPR about this as well.

Part of it might be that WWI only had a a couple of decades to take hold in the memory before WWII came along, and WWII has had 70 years to marinate in the collective memory banks.