No, marketing is not the big problem. The lack of compelling software for traditional gamers in the broader Japanese market is the problem. Look, what Segs and Charlie highlighted is exactly right -- the DS's successful sales foundation was built upon its ability to appeal to three core demographics: the casual (and ex) gamers who found the interesting new types of software appealing; the Nintendo gamers, who were enthused by their same old Nintendo franchises; and gamers, who were appealed to by way of all the other great and promising stuff for the DS (Square Enix's heavy support, Jump, the adventure games, and so on et cetera).
This has not been the case with the Wii, which was pushed from day one to the casual gamers above all else. From its presentation (TGS, E3 07, Nintendo's various shows) to its game lineup, that's all Nintendo has stressed, getting those casual gamers. At some point they panicked and realized they needed the Ngamers, too, which is why we're seeing Mario Party, Mario Kart, Mario Strikers, Mario Stadium, Mario the Doctor, and Mario at the Olympics... but even then, they've failed to reach out and appeal to gamers writ large, and as far as a leading console goes, that'll be their undoing. Nintendo hasn't even made the slightest effort to rope in (via games) the people that made PS2 software successful.
And this is Nintendo's problem. They didn't make the right incentives (whether monetary or otherwise) to bring third parties on from the start to make the games they couldn't make and have them there during this critical first year period, and they didn't make any effort to broaden out their own software development efforts -- despite the fact that they had all the time in the world (a good two years as lead time) and the easiest developmental platform available (one using all but identical programming structures to their last console). Man, it's amazing how Sony gave them the absolute most perfect opportunity when they priced the natural successor to the PS2 out of the market, but Nintendo just couldn't follow through by providing the games that would make people want to jump in.
Oh well. Too bad, so sad.