The entertainment industry now tends to sell blockbusters (ie. Top 40 pop) or niche products (ie any of the many genres mentioned) without much of a middle ground that is likely to develop into something more mainstream.
Yeah, that's kind of what I was looking for. When you think of all the mutant strains of dance music that have evolved over the years (Psy Goa Garage Trance, anyone?), you'd think one of them would have broken out. I guess trance did to some extent, and techno had its moment in the sun. We have a lot of sub-genres for sure, but I'm thinking of something like disco, which kind of came out of nowhere.
Another possibility is that we are exposed so much to a rich tapestry of all kinds of music and permutations that we have all we need to have a 24/7 satisfying musical experience. We don't really need anything "new" because there's so much from the present and past we haven't heard yet, and would seem "new" to us.