I'm thinking it's because it's going to launch with the NX and they couldn't reveal details of the system at the time.
Going to agree with that, mostly because it seems to be the most logical conclusion, but I hope that we don't see the NX until late 2017/2018. Maybe Nintendo can learn from the Wii U's failures, but I highly doubt they are going to.
I'd actually like the see details about the NX being revealed next year, especially after the paper-thin amount of content that Nintendo had at E3 this year. Not sure what the NX would look like though. How can Nintendo surpass Sony and MIcrosoft's consoles whilst also appealing to their handheld market?