What was the reason the vision of 40 acres and a mule turned into a disappointment in "The Souls of Black Folks"?
This "vision" was really a promise made to freed African-American slaves during the Civil War. General Sherman, in Special Field Order No. 15, said that some 400,000 acres of land that had been confiscated in the South during the Union victory should be separated and deeded to the freed slaves to take as their homestead. Sherman doesn't specifically mention the mule, but the armies pack mules were certainly available and it is possible that the government handed them out. Whether they did or not, history remembers it as being part of the promise.
The promise was quickly revoked, however. After the death of Lincoln, Johnson revoked Sherman's orders. Any land that had been settled by African-Americans was returned to the white former owners.
As a result, this phrase - as described in "Souls" - has come to represent the failure of the American government to protect and assist African-Americans as they tried to intregrate into American free society. It is also the failure of Reconstruction. These failures, as summed up by Dubois in the introduction, causes "the problem of the twentieth century is the problem of the color-line."