I'm not sure why, but it seems to me that Nintendo never really was very much interested in marketing. I think they probably figured it didn't make much difference to the overall success of a game. It seems back during the Genesis-SNES era that Sega spent a lot more on advertising and they came out with some "cool" commercials which appealed to the hardcore teenagers of the day, but I can't really remember any Nintendo commercials from that time.
Nevertheless, Nintendo did win the 16-bit era despite their poor marketing so maybe that was what gave them the idea that marketing doesn't really make much difference. And they also won the handheld war of that era too. Sega did a good job of pointing out how their gamegear had color and that the gameboy was as bland as spinach, but Nintendo won that war as well.
I think what it comes down to is Nintendo thinks word of mouth is a better way of selling games than with advertising. They probably think making a high quality game with minimal marketing is better than making a low quality game with heavy marketing. As the saying goes, cream rises to the surface and crap sinks to the bottom. Marketing can help a crappy game in the short term, but only a high quality game can be a long term success.