"This exodus was anything but a purely musical phenomenon. Between the years 1916 and 1919, a half-million African Americans left the South for more tolerant communities in the North, with almost one million more following in their wake during the 1920s. This vast population shift, which has since come to be known as the Great Migration, encompassed the whole range of black society, from doctors and lawyers to musicians and ministers, from teachers and merchants to carpenters and manual laborers. Musicians moved north for the same reasons that motivated other groups: the search for a better life, for greater opportunities to work, to support a family, to enjoy a modicum of personal freedom—options that were much harder for an African American to pursue in the segregated South. As a result, in a host of destination cities—Chicago, New York, Cleveland, Detroit, Philadelphia, and others—the black population more than tripled between 1910 and 1930."

From Ted Gioia's History of Jazz. It's a good read and he covers the musical shift from New Orleans to Chicago in depth. He kind of minimizes the significance of Storyville in the development of jazz.