Much of pre-modern music is written around a set of rules (which ones was probably not agreed on by everyone) which a lot of it stuck to.

There were some who broke with them in some ways and it wasn't as strict as I'm making it out to be. But Jazz just sounded radically different from everything known at the time.

Chances are this didn't matter either way, Jazz was considered degenerate because its pioneers were black, end of story.

A lot of the objections were explicitly racist, even if the musicians were white. The social origins of jazz were also important as it was associated with poor urban culture. Finally it was associated with dancing and sensuality.