I don't see why a fighting game can't be "uglied" down and released on the Wii as well as the 360 and PS3. Just reduce the polygons of everything to a workable level. It isn't going to make the game unplayable, but for the case of Soul Calibur IV, it may mean that there will be less room for bounce. Anyways, the exact same play experience is available on the Wii as on the PS360, regardless of look, especially for a fighter.
My guess is that Namco will release it for the Wii at some point in time, perhaps if they can get Nintendo to loan them another exclusive character to ensure success. Either that, or the fifth game will be a Wii title. I think the fact of the matter is that developers, or at least publishers, are idiots a lot of the time. They don't know who wants their games, they don't see industry trends, and they don't pay attention to what's best for their franchises most of the time. They just are idiots, plain and simple.
Now, nearly every publisher believed the Wii would flop. That's a fact. American and Japanese publishers alike probably saw it as a Nintendo swan-song for some idiotic reason. They weren't planning on making anything for it ever, instead riding on the Playstation name that was the real flop any industry-minded gamer should have seen coming at least by last year's E3, but probably earlier, and no, I'm not kidding about that. So the publishers were blindsided for no real reason other than their own flawed thinking upon the true start of this generations war, when all three systems finally were released. So what do they do? If you ask me, most publishers now have freaked out. They don't know what to do or what to make, nor do they know what platforms to make it for. They still haven't realized that they can still make multi-platform next-gen games for the Wii, PS3 and 360 all at the same time. I bet the devs know this is possible, but the publishers won't let them because they really are idiots in most cases.
In this case, I bet SC's devs know that the Wii is a quality system, but Namco's publishing division won't let them for fear of some unreasonable thing. I'm sick of it too, and I'm on the verge of realizing that more than half the people in control of the video game industry are completely backwards from how they should be. Sometimes I wish I were in charge of some big publisher, just because I'm almost completely sure I could do much better than say, Konami, Namco, or Capcom. I don't know what I could do to save Sony for this generation, though.