For those of you who might be unaware, CAPCOM is a Japanese game studio that brought us the Resident Evil, MegaMan, Devil May Cry, Viewtiful Joe, and Street Fighter series (Not in that order). Street Fighter 2 single-handedly transformed the fighting game genre, and got people playing against each other instead of the machine. It had a depth of strategy and tactics, eight different player characters, and special moves unique to particular characters that differentiated it from the other games out in a big way. Street Fighter 2 was a big hit for the Super Nintendo, and also later for the Genesis. Street Fighter 3 was a big hit in arcades among fighting game fans, but it didn't really set the home market on fire, as it was only available on Sega's Dreamcast console at the time and only came out on Playstation 2 years later. The other problem with SF3 in terms of mainstream acceptability is that they added too many new characters and dropped too many favorites.
Street Fighter 4 is out in arcades now, and wild speculation abounds about the home versions in terms of what characters will be added to home versions, and what machines it will come out for. The official line from CAPCOM is that it will come out for PS3, XBOX360, and PC. While some interviewers got some CAPCOM employees to say things that were Wii-positive, no official statement from CAPCOM includes the Wii in the SF4 lineup.
I'm somewhat surprised by this. The arcade version of SF2 and the home version that came out on the Super Nintendo were not exactly the same. One of the bonus games got cut from the home version, the character sprites were smaller, but the essential parts of the gameplay were left intact. I don't think it's reluctance on CAPCOM's part to work with Nintendo, since they have two reasonably successful Resident Evil games on Wii right now, they're working on a new Monster Hunter game for the Wii and they were even willing to try out new properties (Zack & Wiki, We Love Golf) on the Wii.
So, I can see the reasons that CAPCOM doesn't want to port Street Fighter 4 to the Wii. The first thing is that a Wiimote/Nunchuck control scheme is awkward for a six button game, and I get the impression that Nintendo won't let a game out the door if it doesn't have a way to use the Wiimote. Mandating that people use the Classic controller may be a losing battle, especially since Nintendo's still involved in a pending lawsuit about the analog controllers in both the Classic controller and the Gamecube controller.
History is also against us, since the last time a Nintendo system had a real Street Fighter game on their system it came on a cartridge. Yes, I know, there was Capcom vs. SNK 2 for Gamecube, but it was released much later than the PS2 version - so a lot of the people who wanted to play it already got it before the Gamecube version came out. Most reviewers panned the Gamecube version when it did come out because they felt the dumbed-down special moves in the EO version broke the game.
So, perhaps it is a problem of demographics. Fighting games are hardcore, Wii is not. People that want hardcore fighting games already have one of the other systems. I would be ready to believe this if it were not for this quote from Yoshinori Ono being interviewed by Russ Fischer of chud.com about what they had changed in the fighting system from 3 to 4:
Just playing Street Fighter 3, you could tell it was a game made by people who love fighting games for people who like fighting games. It's a specific audience, really. We made the rules so rigid and severe, so technical and deep, that if you look at making fighting games like a mountain, we pretty much made the peak. There's nothing else up there but air; you can't top SF3 when it comes to complexity and depth. So we decided to go a different direction.
To use a chess analogy, which we like around here, chess is the same no matter where you play it, and Street Fighter is Street Fighter, whether you play 2 or 3 or 4. But a really high level chess player is in a tournament with a timer, you can't take back a move, etc. That's SF3. The hardcore, high pressure chess. The other end of the spectrum is old men in the park playing on a cardboard board; they're letting each other take moves back, they take an hour to think of a move with no timer, they have weird house rules, whatever. That's street fighter 2.
What we learned from 2 and 3 is that, while 3 is a great game and we hope you still play it, for 4 we wanted to take it back to the fun of SF2, to the guys in the park instead of the pros.
So, what I see there, is mainstream, not hardcore.