The emphasis in jazz had been to use a song, with a circle of fourths type chord progression, and a 12 or 16 measure phrase as the basic structure--with the advent of Be-Bop, the rhythmic/melodic phrase became more important, and the solo possibilities opened up--the thing was that the circle of fourths chordal structure, and the movement from tonic to dominant tended to dictate too much where the soloist could go--"modal" allowed the soloist to use a single chord(or short pattern of inversions and substitution chords) as the basis, and to extend it or play against it with a variety of modes or altered scales--the soloist also had the option of playing phrases of almost any length--