I'd say the biggest reason why there are few M-rated games on Wii is because Nintendo rarely publishes an M-rated game. Since they've invested the lion's share of money into Wii software development, that means there are relatively few M-rated games.
This isn't the same as the OP's reason number 4. It's not a deliberate attempt to 'keep the Wii clean,' Nintendo simply doesn't like to raise barriers to a game's purchase by having high ratings. As third parties invest more in developing Wii software, we see more mature titles like House of the Dead: Overkill and MadWorld show up.
"The worst part about these reviews is they are [subjective]--and their scores often depend on how drunk you got the media at a Street Fighter event." — Mona Hamilton, Capcom Senior VP of Marketing
*Image indefinitely borrowed from BrainBoxLtd without his consent.
