As part of his end of financial year briefing, Nintendo’s president Satoru Iwata has spoken on how Mii for use in Wii games now must go through the same process of licensing as the Mario Characters do for use in third party games. Iwata says this is to protect not only Nintendo’s image but the Mii’ brand, for example Nintendo probably wouldn’t allow 3rd Parties to use the Mii’s in anything violent. Could the cost and time taken by the licensing process be why so many third party games haven’t used the system? .
Source: NeoGAF