Capcom producer Tomoaki Ayano took questions from the press following the announcement of Ultra Street Fighter IV. Of course, the big one in everybody's minds was, why didn't they make Street Fighter V? And to this Ayano provided an answer.
As Ayano points out, each iteration of Street Fighter took a long time to develop. There was a six year gap between Street Fighter II (1991) and Street Fighter III (1997), and then a nine year gap from III to Street Fighter IV ( to clarify, nine years from SF III 3rd Strike and SF IV in 2008). Ayano expects Street Fighter V to be ready by 2018.
On this point, I would add that Street Fighter itself came out 1987, so there was a four year gap between it and Street Fighter II. Also Ayano wasnt really able to elaborate on the circumstances that led to the gaps between each sequel. For example, Capcom dabbled in the Alpha and Versus series of fighting games before getting into Street Fighter III. SF III itself did not perform as well as Capcom expected, and so the series was put on the backburner for an extended period, waiting for current series producer Yoshinoro Ono to revive interest in the franchise.
In the meantime, Capcom made Ultra Street Fighter IV to refine IV and grow the fanbase even further. The new characters themselves obviously come frome Street Fighter X Tekken, but as Ayano points out, they will not be simple copy and pastes from the latter. The two games use the same core graphics engine, but play very differently, and so the new characters will be adjusted accordingly. In particular, their Boost Combos will be replaced with SF IV's Ultra and Super Combos.
Capcom's decision to improve Street Fighter IV is a reflection of them listening to their fan community, and is also in line with their history of improving each game. Capcom must already be targeting Street Fighter V for Playstation 4 and Xbox One hardware, so give it time, that game is going to be awesome when we get it.
Source: VG 24/7