It makes sense to me that Xenoblade would sell better in NA, if not for the design of the game then because NA has a larger Wii userbase.
I think you're selling the statistic short.
This is a game that came out when the Wii was pretty much dead in the water and only had limited copies sold at one major chain (GameStop) and online. After which it pretty much disappeared from everywhere except Ebay. It had pretty much no marketing to speak of, and launched on a console whose audience tended not to be interested in these sorts of games.
For the game to sell better here despite all of those factors is frankly kind of amazing. NOA should be thanking Operation Rainfall for recognizing (and perhaps even creating) a demand that they otherwise likely would have ignored.