I think the last post got it bang on.
I was thinking a little more, and it struck me that the absence of community in the physical sense is the limiting factor.
If you interpret road rap as partly an evolution of the grime scene (similar actors, customs, aspirations, methods etc...) then what strikes me is the lack of interplay between people in the scene. There is no real scene, there is just a collection of individuals doing similar things autonomously.
When it was grime, and the internet thing hadnt kicked off to the extent it has today, people were referencing each other, calling out names on the track, subliminals, postcodes, clubs (i never went to stratford rex or the palace pavillion, but those names are ingrained in my imagination) clashes etc... This cross pollination of ideas, challenges was what fuelled the communal aspect of the sound, the scene as it were.
Now with road rap, especially in its most stunted, reactionary guise, that tradition of referencing other people, other crews, other areas seems to have gone. Almost all the videos on youtube i see are purely about promotion of the self, or the small nuclear gang unit - not about promoting a sound, a movement, or even a lifestyle. Its become so introspective thanks to the medium it operates within and i think that's part of the problem.
The counter argument is that increased musical exposure via the web necessarily leads to an enriched musical heritage and increases the chances of weird hybridizations and sonic mutations ocurring. But if they occur in a bubble, in isolation, or in very small circulation, are they really influential at all?