Well limited advertising, lot of people were angry it wasn't RE style (mad about it being on-rails), favorable but not specatcular reviews for great word of mouth, and just general disillusion with the wii userbase and the title. Did worse than even I thought it would but I never expected it to do well either.
I'm most curious that why everytime a Wii game does poorly it deserves discussion... every platform has it's successes and flops. Either discuss them all or don't discuss any of them less you want to have a contradiction.