As far as I can see, there are two reasons.
A) Nintendo fans are ... passionate about Nintendo. They use words like "charm", "heart", and "whimsical". They have an emotional connection to Nintendo. This means that they will click on Nintendo themed content. Good, bad, ugly, it doesn't matter--if you write it, they will come. That equates to advertising dollars.
2) Nintendo finances are easy to understand. If their gaming product does poorly then they're in trouble. That's incredibly simple to comprehend and it's related to discussions most gamers can talk about semi-coherently--video-games. Microsoft and Sony finances deal with a lot of other stuff, most of which the average gamer is going to know little about. For example, I read a user comment in a Yahoo article not long ago that said, "the Xbox One will be better than the PS4 because Sony is just a TV company while Microsoft has been building computers for decades".
