There's a number of reasons.

1 - Zelda has a MUCH bigger fanbase than GT games, Zelda has been around a lot longer, covers many gaming generations.

2 - The racing genre has been dying. People want more out of their games, you can only do so much customization in a game. The rest is all making turns and trying not to crash. Its becoming a smaller and smaller niche market. Thus why you don't really see many developers making racing games anymore.

3 - Link as an IP is among the top 10 in videogame history. GT doesn't really have a specific character that draws people in. The Zelda IP and the character Link are sort of like a book or tv series - you want to know what adventures the main character is going to go on next.

4 - Zelda games are very popular across both sexes, where there traditionally is a small amount of females into racing games.

As for why the full year? The game releases in Japan in Jan/Feb barring any delays. Releases in March in the US. Sales will stay afloat a few months, with a natural summertime lull, and then the demand will be very big once again come holidays 2011, so June-Sept might be a bit slow, common for the overall market. Yet the initial Zelda demand will get those hardcore Zelda fans who havent yet got a Wii (yes, they exist, surprisingly!) out to get their "official" Zelda Wii game.

I dont disagree that it might be down YoY, my point is that it'll still have enough spark to win the holiday sales battle over the HD systems again next year. Come 2012, all 3 will be drastically down, as I fully suspect new home systems to be announced in 2012 for the 3 companies. I just feel that of the 3 Nintendo still has that one more big push (Zelda), where I feel that the HD systems have already thrown their full arsenal into the arena already.