If I'm being honest with myself, then yes, it's because it's a Nintendo game that Splatoon is receiving such attention, or at least mine. Had it been, say, in Sunset Overdrive's position on the Xbox I don't think I would have been as excited for it.
But I believe part of it isn't so much biased in favor of Nintendo games overall (though that's still a factor), but that this game coming from them is intriguing for several other reasons. Nintendo has insane quality standards for their games, so I don't doubt this will be a solid game that's polished to near-perfection. It's also a new IP from Nintendo, which always seems to garner excitement, and a genre they really haven't attempted to any significant degree and the premise and gameplay looks fun and unique. Then there's the fact that it's the product of Miyamoto's proteges' work, so it's interesting to see what the new blood can come up with.
So yes, I would say it's getting the level of attention it is because it's a Nintendo game, but not entirely for shallow reasons.