Because its probably to late. Money/time constraints. Finding enough people to work, paying for labour etc. Maybe its to late to ship... Plus they need to get a copt of Wii Sports for each Wii. Maybe they'd prefer to not do them all at once incase there are problems.
I mean, honestly, dont you think they would have thought of this? I dont know anything about what goes in to making a console, but i'm sure atleast one of the above reasons would be true.