1: From what we've heard so far, it will be underpowered by running with Nvidia's mobile Tegra processor which makes it even less powerful than an Xbox 360 and third party developers would want to avoid a console that has an entirely different architecture than its competition, that was exactly what happened during the Wii and Wii U era.
2: Lack of online gaming and the fact that it will use gimmicks and run on cartridges, that was the same reason they failed with the N64 and Gamecube and it'll happen again because they didn't use a decent optical disc format that can store big games.
3: It will likely be another PR flop. It will be revealed in a time where E3 is over, if you want to cater the decide to the masses then you have to do that by promoting it as much as possible, by not doing that they are showing that they only want to sell it to a specific audience, which is an obvious indication if you only promote it a Nintendo Direct. E3 is a brilliant place to create hype and promote a console; you let people try it, look at it and you create hype, all that is not there if you're just gonna make a cheap Nintendo Direct, which only a specific install base would know existed.
4: The NX was an idea brought to life when Iwata was CEO. Nintendo are getting more and more irrelevant by following a pattern where they sell stationary consoles at a decrease every gen, by not showing it at E3, it's obvious they are afraid of it getting overshadowed, which could mean they are admitting that it won't be a good console and that it will fail even more than the Wii U. It wouldn't surprise me if the NX actually sold worse than the Gamecube and Wii U. The Wii U sold most of the consoles during the first half year which was about 5 or 7 million where it was on level with the Wii 1, but after that it was done and it's only going to get worse and worse. They're selling at a decline every gen. If they're already releasing the NX in in 7 months then it most likely won't even be a next gen console but a current gen console and it would sell worse than the Gamecube, because the Gamecube was only released 1½-2 years after the PS2, but the NX is releasing almost 3½ after the PS4 and Xbox One which already has a lot better hardware, a ton of games by then and low prices compared with the NX.
5: Think back of the Virtual Boy and N-Gage which were both hybrid that failed. Face it, hybrid is a terrible idea, because that way no third party would support it unless it becomes a success, but even if that was the case it will be the same case with the NX as the Wii 1 and Wii U where they get problems by only releasing a AAA title every 2 or 3 years. The NX will be a flop, something like the Virtual Boy where they're not sure to decide whether they should call it a handheld console or a stationary console. Nintendo are much better off as a third party publisher, Pokemon Go is a proof of this, Sega is another proof of this.
6: Very few poeple buy handhelds anymore. Hanheld console sales recently have been at a decline for years and the idea of a stationary and handheld hybrid couldn't become anything but a failure.
7: Weird time to release it. They will release the thing in March, 3 years after PS4 and Xbox One in a time where people far from buy as many games as Christmas and by the time it's Christmas 2017 the NX will be crushed by the competition.
9: It will miss out on a lot of genres that caters to the masses like RPG, FPS, Sandbox and Racing. If they can't fulfill all those genres and design a console that has as much a broad install base as possible, then there's no way ever they are going to succeed with any of their future consoles.