Nah, it's not as simple as that. A lot of it had to do with console fatigue, which Wii faced much quicker than previous consoles due to being last gen tech. Galaxy 2 is also at 6.4 mil while Galaxy 1 sold 10+ million. Neither of those are numbers to scoff at. Mario is definitely easier to market and, in a lot of ways, easier to make than zelda games. Mario is about pure unadulterated platforming bliss, and while it's fantastical visuals definitely made it very appealing, presentation is much more important in a game like zelda. Nintendo found a way to get around the lack of massive scope with Mario, which is why its gameplay never felt dated throughout this entire gen. Zelda on the other hand, while still an awesome experience, feels cramped and claustrophobic in comparison to other adventure games.
I'm tellin ya, zelda, more than any other nintendo franchise, is going to benefit most from the jump in processing power.