In the USA? I can give a few reasons:

Viva Pinata - It's *gasp* casual! Not Microsoft casual (ie, poor selling), but Nintendo/Wii casual (ie, evergreen). Viva Pinata 1 debuted at a whopping 10,690 units in North America 2 years ago, when Gears launched. It wound up selling 600,000 units without bundling. VP2 sold 3.5x that many units in North America for it's debut week. Does this mean that VP2 will sell 2 million units in the US? Who knows.

Infinite Undiscovery - Unfortunately, it looks like North America is (sadly) reverting to it's pre-PS1 days of shunning JRPGs. No JRPG has yet to get traction in the US of A, regardless of system, outside of maybe Lost Odyssey. I can give a laundry list of games that haven't done well. But the fact is, unless your Pokemon or Final Fantasy, your sales are going to suck for a JRPG in the US. Western RPGs are just so much more appealing for some reason.

TWEWY - 264k

FFT2 - 241k

FF:CC ROF: 201k

FF:CC (PSP): 576k

FFT:TLW - 109k

And so on. The biggest selling non-Pokemon JRPG this year is Final Fantasy: Crisis Core on PSP. Whoop-de-friggin-do.

However, to be fair: JRPGs aren't front loaded in the USA. I don't think your going to be able to make a valid opinion on first week sales, since some JRPGs have had great legs after horrid debuts.

Viva Piñata is a micromanagement hell of resource management. I'm not sure I'd call it casual but rather a combination of good word of mouth + low price.