Kotaku has reportedly spoken with insider sources at Nintendo, who claim that the lost exclusivity of Resident Evil 4 caused Nintendo to reconsider the inclusion of Capcom characters in Brawl. RE4 was originally meant to be exclusive to the Gamecube, but concerns over sales led Capcom to release the title on PS2, and later on PC, with additional content.
While Nintendo has the right to be peeved about the broken exclusivity agreement, the addition of Capcom's franchises to the Brawl lineup could have only benefited Nintendo, and created more hype for the title (if that's even possible). While we're a bit unsure whether to buy into this rumor, if true then we can only hope that Capcom and Nintendo kiss and make up in time to offer some new characters as downloadable content (hey, we can dream, can't we?).
Update: Slight factual error corrected; Capcom announced the PS2 version prior to the release of the Gamecube version. [Thanks, 2bit]