Realistically speaking a decision to delay a game like Smash Bros. is never made without looking at multiple factors ... If the Wii did not have Super Mario Galaxy, didn't have as large of a third party line-up, and wasn't selling as well as it is I'm pretty sure that Nintendo wouldn't have delayed Smash Bros.
Now, the reason to delay Smash Bros. was probably because there was doubts about whether the game would be completed, tested and bug free in time to make the holiday season; early December was pushing the envelope anyways and the delay of a week (or so) would have lost a lot of sales. By pushing it out until early 2008 they don't have to worry about how many bugs are in the game they're releasing and they (probably) don't lose any sales for Smash Bros. or the Wii over the holiday season.
