It's also probably that:
1) The games haven't been ready to show
2) They're super worried about a Wii U repeat of a year+ wait after reveal
3) The NX will likely play Nintendo's iOS games so having, you know, famous and popular iOS games will make that reveal make more sense
4) The Wii U has no winter so they're not distracting from anything there but they also have to be real careful now not to distract from Sun/Moon
Hehe I'm glad I'm not the only one that thinks it will play mobile games. (You have to give the mobile demographic some convenience to have one system that plays mobile games and traditional games. Especially in Japan.)
I'm not concerned about distracting from Pokémon S/M. If they reveal NX now, it still gives 2 months to ramp up marketing for the Pokémon games which have been shown since the 20th anniversary. Even Pokémon Go is having a "spillover effect" which as increased sales of other games in the series as described in that WSJ article yesterday.
Edit: I'll list some assumed benefits of playing mobile games on NX compared to an iOS or Android device.
- Firmware updates not breaking games like Capcom's games on iOS.
- Games running on Vulkan maybe? There's supposed improved performance that the games run longer with regards to battery life.
- The device shouldn't be hot to hold while playing.
(These are all assumptions so I could be wrong.)