I doubt Mario 3D world is going to have a bad launch.
Super mario galaxy also had a low first day sellthrough (around 130k to a 23% shipment sellthrough) and sold 250k in it`s first week.
Mario 3D World has 20% sellthrough in the first day, so sales could end up being closer to Super Mario Galaxy`s first week, despite the difference in userbase.

I never understand the legs on games like this.  Clearly the series has an established fanbase, but it seems they just aren't interested in buying the game day 1 and would rather wait for a price drop or will just pick it up whenever they feel like.

I mean, I understand that'll happen to a certain extend, but to happen in such large numbers?

Is it more to do with the audience being skewed younger possibly?

You've hit the nail on the head - younger audience including many children means they tend to become aware of these games more gradually as they come into contact with more people who own it and like it, or as they begin watching the anime whilst looking for something on TV. Same reason Youkai Watch has shown such good legs. There is of course a smaller core of fans who buy it as soon as they can, but on the whole this just seems to come with the territiory when selling games to kids (predominantly)

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god damn Dragon Quest is popular, you could put it on Xbox and it would still sell

If in 2 days there's still no Zelda... I'm going to eat my hat.

No! They put Monster Hunter on Xbox

yeah but that wasnt a real MonHun, if DQXI was an XBO exclusive next year the XBO would fly off of shelfs in Japan

exclusive Monster Hunter. Xbox got a boost and died again. I don't think DQ can do something for Microsoft in Japan

Zelda games don't sell well in general in Japan these days. Skyward sword merely sold 360k there and WW HD didn't do so hot either.