Because if it can't do everything else my current tablet can, I'm gonna have to carry a tablet AND this thing. Done right, Nintendo can make this thing the first real challenger to the iPad for mass-market adoption. But they've gotta treat it as a first-party Android device and get updates out ASAP and not muck with the interface too much. I'm willing to bet they could work out a rev-share agreement with Google on the Google Play store and Google Play Apps (and keeping their own Nintendo licensing scheme).
But let's not kid ourselves here: Nintendo is a Japanese company and it operates like one. That means they'll try to own the entire value chain and miss out on any network effects, while simultaneously moving themselves from a market with a 5-10 year refresh cycle to one with a 2-3 year refresh cycle. While it means they could sell more tablets to repeat customers, it also means that they have less time to be patient for success (as happened with the Wii and WiiU) since it also increases customer churn. Network effects and platform lock-in are a lot more important when the refresh cycle is shorter, because there are more opportunities for your customers to jump off the train.
I wish Nintendo luck, and I think that this is a good usage model. But I'm not convinced it's compelling enough to displace the tablets that people are already carrying around with them unless it can also duplicate the capability of those devices.