My theory is that the web has overall matured. A lot of what Flash did is now possible and even better/faster/easier, but the reason for those aren't there. In many ways Flash websites featured novel navigation and interaction experiences, the likes of people never had seen before, but that's a gimmick that gets old fast. No insurance company wants or needs an animated 3d intro in their website; they want and need good SEO, and information that's easy to reach. If they want to impress with something visual, they'll go to YouTube or Instagram.