One simple word - Tradition. And I think they pride themselves on making fun games that do not need to be graphically intensive which is the biggest reason I personally like them.
Trust me, I like them a lot because of that too. Their coop games are JUST AWESOME when you have friends/family/relatives/cousins coming over and you're gonna have a BLAST playing awesome games with them.
It's just that, I think the other developers are flat out abandoning the Nintendo consoles is coz Nintendo consoles aren't powerful enough to handle the current gen games.
Listen to this, if you were a developer, wouldn't it just make sense to port over your game to an equally powerful and easy to code console so you can get extra money out of your current investment?