Simply because something is profitable doesn't mean it's a worthy investment.
I tell you what, give me $1,000. In two months, I'll give you $1,001. You just made $1 profit, so that was worth it, right?
"ROI" - or "Return on Investment". It's about maximizing your profit, or return. Nintendo could spend $1,000 marketing Xenoblade or they could save that towards marking the 3DS, a particular 3DS title or even towards the Wii U. Meanwhile, the same crowd is likely to buy Xenoblade - the funky sounding game with a gameplay style no one likes anyway - and Nintendo just blew their entire advertising budget (that $1,000) for the year.
If Nintendo of America's "proper backing" pushes a game just barely above 600,000 units... well, that's pretty pathetic. Who was it that was freaking out because Skyward Sword only sold ~600,000 units within its first week on the market? Now, we're suddenly supposed to be throwing a parade for a game that managed to barely squeak that much during its total life span on a system that has a far larger install base?