Here's Christian Svensson's comments from the Unity Boards on why Tatsunoko vs. Capcom is only being released on the Wii this winter.
Why was the Wii chosen for Tatsunoko vs. Capcom instead of the PlayStation 3 or XBox 360?
Sven: [The game] was originally based on an arcade board that was effectively, Wii hardware. So bringing it home to Wii makes the most sense.
Will Tatsunoko vs. Capcom only be on the Wii?
Sven: I'll say it again: Wii only.