NintendoWorldReport says...The urge to push the sequel "as far as it could be pushed" kept the game off of Nintendo's console.
After two major success stories on the Wii, the Resident Evil franchise is skipping Nintendo's console for Resident Evil 5. According to Producer Jun Takeuchi, the reason why is that the team "really wanted to push the next part of the series as far as it could be pushed."
As part of an interview with Joystiq, Takeuchi was asked why the game wasn't coming to the Wii despite Nintendo's market dominance, cheaper development, and the success of Resident Evil 4: Wii Edition. Takeuchi explained that the decision came "from the development team." Resident Evil 5 would arrive on the Xbox 360 and PS3, and not the Wii, because of the team's ambitions, including those for "the graphical aspect of the game."Takeuchi also stated that "actual development time itself has been just about three years." Including planning, Takeuchi put the figure closer to four years. The Wii in comparison is only a little over two years old. When Resident Evil 5 was announced in mid-2005, the Wii was still known by the codename "Revolution," technical details were still scarce, and Nintendo's console was still over a year away from launch.
Source: NintendoWorldReport