remember Wii motion plus wasn't born till after the Wii's lainch, by that I mean that it wasn't being implimented in the development of games till more recently
so this means that they had to have restarted development on the game between the initial beginning of development and the time they decided to impliment Wii Motion Plus.
I'm not saying they totally trashed everything they had worked on uptill that moment, but that they had to have done something along those lines.
so i think that's why it's been in "development" for so long.