The answer to that is poor working conditions and cruel treatments of peasents.
Powers not delegated to the U.S by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.
So basically, they are reserved to the states and left up to them to decide.
Two of the expected demands made by the Treaty of Versailles towards Germany are 1. Germany had to take sole responsibility for the war and 2. Germany had to lose much of its industrial areas.
Even if the United States had wanted to enter the war, its military force was simply not ready.
