When you compare and/or contrast the Exodus of 1879-1880 when an estimated 40,000 to 60,000 African Americans relocated to Kansas with the Great Migration during the interwar years when an estimated 1.5 million Black Southerners headed to Northern cities such as Detroit, New York (Harlem) and Chicago, what explanation best captures the motivation for both historical movements?
In both cases African Americans were fleeing harsh and oppressive conditions in the South. The first period you mention 1879-80 was shortly after the reconstruction ended. White southerners were essentially trying to re-enslave the black population without calling it slavery. Black laborers were forced to work under highly unfair conditions and threatened with violence if they resisted.
In the period after WWI, there was a much larger exodus from the South. Again, African Americans were fleeing harsh and unfair conditions in the South where whites were still trying to force the black population to work for them under harsh conditions and threatening violence if they resisted. The movement this time was much larger for several reasons:
Many of the earlier legal barriers to movement had been removed. Attempts by white land owners to keep blacks on the farms through use of debt obligations and other restrictions were less effective.
Transportation systems made movement much easier.
Larger cities after the war had much more demand for labor, offering better jobs than the harsh conditions of farming on the Kansas frontier.
An existing African American population in many northern cities made those locations more attractive to southern blacks.
Many African Americans had been exposed to the better conditions as a result of military service during WWI.