Because it's not usually a good idea to fit your deck into a certain frame and give it just a certain win condition, this especially applies to the Runner's side. A deck and it's player should be able to adjust it's playstyle during the course of the game. If you spend too much effort R&D locking the opponent, he can still be able to outplay you.
Virus Breeding Ground helps you a lot when you can get into R&D. It doesn't, however, help you when you can't. Also, it's usually not that great to spend the whole of your turn running R&D for medium counters, multiple turns in a row, just as it is not that smart to use Melange for multiple turns in a row.