In a recent interview with Famitsu, Street Fighter Producer Yoshinoru Ono stated that Capcom will not make a sequel to a game unless it sells 2 million copies. It came in response to a question about a Street Fighter X Tekken game which won't see a sequel due to only selling 1.7 million copies. His statement was

“For instance, if a game doesn’t sell over 2 million copies, then we’d have to put the brakes on any plans for a sequel. All that means is that we weren’t capable enough. And all we can do after that is to reflect on the experience, take what we can learn from it, and try to apply those lessons on some other title.”

The entire interview can be found over at Famitsu's main page. It's in Japanese initially but you can translate to English. Unless you understand Japanese that is.