Whenever anyone talks about the evolution of music it is important to both consider them as for-profit products, and the consideration of the general public. You don't compare apples to apples between Mozart and Burt Bacharach. "Success" is a measure that must be taken into consideration.
Before WW1 the predominant style of music was Big Band.
This was considered the fancy, party music that you'd hear out of the wireless, in shopping malls, whenever commercial interests were present. Now, this is a time period where each nation, each part of each nation had its own music, so what you'd hear in N'Orleans wasn't what you'd hear in Milano, Globalization wasn't a thing.
The post war period brought an abundance of cheap musical instruments to people who were otherwise destitute. Trying to recreate the Big Band sound with a smaller group, no musical education and no sheet music is what led to the explosion of Jazz.
All throughout the 20s and 30s you'd have a number of competing styles of music that had both their own demographics and their social place; so, classical music was still being played in certain situations, old'timey had its own place, swing is what you'd hear in a dance club, and so on.
If you'll allow me, i will try to avoid the specifics of "what makes a certain type of music THAT type of music". There arew technical differences between Swing, BB, Jazz, Blues, that are relevant to this discussion, but i'd rather not have to write a book.
However one thing that is important to say is that in the evolution of music, two common factors happen constantly: music tries to get more complex, and once it becomes too complex, a simpler form emerges.
In Big Band you'd have these carefully written, carefully rehearsed mini-solos that will become the basis of improvisation in Jazz. Eventually Jazz will self-destruct *because* of the exceptional complexity and a new style will emerge.
Blues is at the root of rock. Three chords, mostly I / IV / V. A solid chunk of blues songs use the same exact chord progression. These 3 chords are made up of either 3 or 4 notes, the 1/3/5/7 of the scale.
Eventually this got boring, and people started adding
tensions, 9 / 11 / 13. Except that the higher you go, the closer you get to the root note. 13 is the 7th of the root, which creates dissonance. Today we appreciate dissonance very much, but it was not always so. Diabulus In Musica was something you could get arrested for in the middle ages, so don't dismiss this as a minor change.
Obviously in the world of classical music this had been taking place already. Schoenberg's pieces are from the beginning of the century, but they were not something that you'd hear broadcast on the radio, but rather performed exclusively as experiemental pieces in private settings.
Tensions also imply that you can exit a chord, and chromaticism is very much at the heart of modern jazz. Well, early modern jazz, we've moved on since.
Blues did have one dissonant note, the "blue" note. That's the passing note between the 4th and 5th of the scale.
For twenty years Jazz took the world by storm. It was both the music of the elite and of the downtrodden - but keep in mind what said before, other styles were still very much happening. Charlie Parker was the symbol of an era, but at the same time simpler styles were still being played, aired, recorded. This is very relevant because the next musical giant will emerge not from Jazz, but from simple blues music.
Here is Confirmation from 1945:
We'll see more of that in about 15 years but for now this is enough.
cont'.