Greg is right -there's jazz and jazz. When I was a trad jazz banjo player and guitarist many, many years ago, everything we played was in major keys, mainly Bb and F, to suit the trumpet,trombone and clarinet. The only tune I can remember that was in a minor key was St James' Infirmary, which we played in Dm (relative minor to F of course and therefore easy for the blowing instruments) A brief foray into modern jazz required far more complex and difficult chords.