Watched an interesting documentary on Miles Davis recently, focusing on his work in the late 60's early 70's. The late 1960's saw Miles eager for musical change and to stretch his music beyond the boundaries of jazz, something that he had basically been doing all his musical career. I don't think Miles even recognised jazz as a consistent musical style, he certainly thought his own music had gone beyond the confines of jazz.
By the late 60's Davis was keen to experience the new musical styles that were emanating from the black musical community, some of these same musicians had been deeply influenced by Miles. Hendrix had been influenced by Davis' bold improvisatorial excursions and this had shaped his influential Band of Gypsies project. Davis in turn was impressed with Hendrix' style, he was also heavily influenced by the new funk driven sounds of James Brown and Sly and The Family Stone.
If you listen to Bitches Brew you can hear the funky setting on Miles Runs the Voodoo down, but you can also detect the influence of the times. The title track has a jagged almost violent opening signalling the troubled period in which the album was made. America at the time was a hotbed of social discontent, especially in the black community where the was a strong sense of pride and self determination, there is no doubt that this ethos influenced Miles and gave his music a darker edge.
Some counter that it wasn't just an artistic or intellectual decision to change is style but also a commercial one. There are some who have suggested Miles was sick of playing small clubs and wanted to play larger more rock oriented venues that attracted primarily a white fanbase. I don't think anyone could begrudge Miles for wanting to reach a new audience, I certainly don't think commercialism was at the heart of this drastic musical change.