To develop a game for WiiWare, the developer has to buy this $2000 development kit thing, and get approved and licensed by Nintendo; Nintendo controls final prices for anything on WiiWare. Nintendo has extreme control over what happens on WiiWare. However, I sorta doubt that one of their conditions for licensing is platform exclusivity- again, World of Goo is a perfect example, since it's available on Windows, Mac, and Linux as well as Wii. It isn't implausible that Nintendo would ask for platform exclusivity, or at least platform restrictions, but it seems like the sort of thing they'd want to do with major developers, rather than a young publishing company like Nicalis, since the major developers' games would be more likely to sell more copies. (Even then, look at how many big-game-company games are available on competing platforms.)
But. The Wii exclusivity wasn't announced until just a few days ago, and you can find articles specifying that NG is to be PC and Wii dating from just a few weeks before. Nintendo would have the power to force this sort of decision, but to do it months after licensing a game seems unlikely. (Unless the Wii version started development two weeks ago, and was in development while negotiations over the license were going on. This seems extremely unlikely.)
That is to say, it isn't impossible that Nintendo made it be Wii-only, but then they'd have also forced Nicalis to keep up the ruse of a PC version for a year, which makes no sense at all.