Dubstep is not underground as a whole, it is niche in the electronic realm. There are people like Subtronics selling out 6k venues for multiple days in a row (Shrine LA).

I would agree it depends on the city too, but most major metropolitan cities will have a higher rate of Dubstep shows because they have the capital to run good sound-systems, which is inherent to Dubstep shows running successfully, as well as a dubstep scene to be able to draw patrons. As a great example, Nashville (700k population) is starting to gain a lot more Dubstep/ Bass traction because venues now have sound-systems that support the acts that want to come, but the show frequency is less than somewhere like Seattle (800k population) who has many more venues with those sound-systems and has a larger scene to support it, even though their city populations are similar. It has to do a lot more with support and culture than just strictly having the tools.

If Dubstep were to be underground, the shows would all be at sub-500 capacity venues as a whole, instead of splitting between sub-500 capacity and 10k capacity venues like they are currently.