What everyone seems to neglect when looking at Zelda's attach rate is its appeal. Those that were expecting higher than 75% are crazy. I think 75% is actually too high. I would have pegged it at around 60%.
Why? Two reasons. First, Zelda is a HUGE name amongst gamers but beyond that you would get blank stares. Mario on the other hand is a household name amongst gamers and non-gamers alike. If Mario Galaxy launched instead of Zelda, we probably would have seen a higher attach rate than Zelda. Especially since it has been 10 years since a Mario title launched with a system.
Two, genre. Zelda is basically an RPG-lite. Not exactly a genre that screams widespread appeal. An easy to play platformer, it is not. Not even every gamer can get into a genre they don't like, and RPG-type games is a tough one to crack if you aren't a fan of the genre. So in comparison, a platformer like Mario has more widespread appeal since it is infinitely easier to grasp and play.
Through the end of the year I expect the Zelda attach to drop a bit as more Wii's hit.