War-weariness certainly played a part, as did a huge war debt and the enormous problems of Reconstruction: reunification of a defeated but still defiant South and consolidation of the vast western territory acquired before the war
Although obtaining land and resources was important to the U.S at the time the cost of war was a real concern. And even though the Union won the the Civil war, they still had to deal with the rebellion of the South. They became violent towards African Americans and secretly owned slaves.