The casual market seems easy to acquire but difficult to maintain because they don't have a committed interest in gaming. Show them something shiny and watch their eyes light up but sooner rather than later, their eyes will wander. I was under the impression that Nintendo wanted to transition casual gamers to traditional gaming. That's why Mario Kart Wii had the Wii Wheel and included Miis. The problem is that some people just don't like videogames or view it as a hobby, only a momentary distraction. Casual interest is sustainable so long as Nintendo can keep providing brand new shiny instead of a sequel to previous shiny. In other words, Nintendo can't just offer more Wii Sports, they have to keep coming up with some else equally as enticing as Wii Sports was. If any company can do it, it's Nintendo but that's considerably harder to do than cater to people who regularly play videogames.
When it comes to core gamers, Nintendo makes many core games but they make them primarily for their own fans. Nintendo chose to cater to them instead of branching out into territory unfamiliar to them. While that's not a bad plan, it doesn't expand their audience. They can try to make a Halo or a God of War but that takes resources away from what they do best. What we have then is a chicken-or-the-egg scenario. Create a new IP to draw in an audience or establish an audience then create a new IP for them.
Personally, I think the latter is a safer bet and it starts with 3rd parties which Nintendo has been making strides in (finally). Before Nintendo has, say, Retro Studios make a first-person shooter for example, there has to be a need for one. That need is more than just the absence of something. That kind of game has to reach more than Nintendo fans because many Nintendo fans will not buy it. Retro Studios could spend 2-3 years developing that a critical darling but commercial failure when they could have made Zelda or another Donkey Kong/Metroid.