I'll bet you anything the whole point of this is to get new buyers, and eShop demos wouldn't help with that. Besides, there will still be the normal eShop demos that become available close to the final games launch.
No need to bet, I specifically said BB was focused for potential Wii U buyers. However, I'm saying they should be equally concerned with current owners and game purchases going forward, especially on multiplat games. eShop demos would definitely help that.
I do understand where you're comming from, but you have to look at it from their point of view. Nintendo isn't going to make much money selling games to 2 million people. That's why they need to grow that install base asap. To do so, they're gonna have to focus more on the billions of potential buyers rather than the 2 million Wii U owners, especially when many of those 2 million are likely to buy Nintendo games anyway. I believe NSMBU has a pretty high attach rate afterall.
Sure they could do both Best Buy n eShop demos, but they already spent much time n resources on something that wont give them anything back, not immediately anyway. Plus having current Wii U owners go to the Best Buy demos with everyone else may help spread word of mouth more since they're knowledgeable about the system and can inform other people there (a stretch I know :L). And again, some of these games will have eShop demos around their release date. No need to spend time n resources making downloadable demos twice when there's better things they could be doing with em.
There's also the possibility of people getting burned out on the demos. This is where you like a demo a lot n want to get the full game, but can't since it's not out yet, so you keep playing the demo over n over until eventually you get tired of it. This is likely why downloadable demos generally release around the full game's launch, so you can pick it up if you liked it's demo. Oh, and whenever I say bet, I don't mean literally. :P