I’m reading two books: the Coltrane book by Ben Ratliff and “Latin Jazz” by John Roberts. In the first Ratliff says one of the reasons the modal thing caught on in the early 60s was because jazz musicians were tired of the restrictions of playing over so many chord changes, and reveled in the freedom of having fewer harmonies. In Roberts’ book, he points out that the reason Bossa Nova caught on in the early 60s, when the more Cuban oriented jazz didn’t, was because salsa typical has very few chords, and jazz musicians enjoyed playing over a lot of chords.
Of course, the fact that they’re saying opposite things about the same group and the same time period is humorous, but I think the bigger thing to realize is that: 1. to take any group of people and try to treat them as a homogeneous mass doesn’t yield true results and 2. I doubt if we ever really know why things happen. At best we’re making correlations between events and convincing ourselves that A causes B. But w/out a controlled study (which you can’t really do w/ real-life events) we never know for sure.