The reason for the Jazz boom in Chicago was the 'Great Migration' of workers from south to north as workers looked for better conditions, terms and status.

Added to this is the fact that Chicago was where the alcohol importers operated from during prohibition - so there was lots of 'work' available in the alcohol 'industry' including a strong subculture of bars etc who employed musicians. Jazz was the preferred tipple of the young.

Venues got bigger and dances filled bigger halls, so in the absence of large PA's the Big Band was born.

This was the next stage or 'mountain top' of Jazz after dixie etc.