The reason? Apparently Nintendo’s President Satoru Iwata is worried that if they revealed it too early, the competition might steal their ideas and bring it to the market ahead of them and thus render the console no longer special. This is according to a post by @NStyles which was then translated by @Cheesemeister3k.
We suppose that makes sense but at the same time this means that the public will just have to speculate at what the Nintendo NX could be, which previously was speculated to run on Android but was later refuted by Nintendo. Iwata went on to state that until the NX was launched, the company would continue to support the Wii U and 3DS consoles.
According to Iwata (once again based on a translation), “The NX is new hardware, and will start from 0. However, the 3DS and Wii U have install bases. Immediately cutting off software for previous hardware upon the release of a new machine is inefficient We will continue making 3DS and Wii software while preparing for the NX. We are prioritizing satisfying customers who purchased the Wii U.”