Too little shown. If they had more I feel it would have had the same amount of screentime as BOTW. How often has it been that we've seen the first footage for such a big game just 5 months before it launches? Nintendo typically takes their time with these things, I don't see a game shown with as little footage as that to come out in March, probably Summer or Fall. It also looked visually very similar to 3D World and I feel as though Nintendo will want to make it look more unique as it develops further.
These aren't solid 100% predictions or evidence, but it seems that accounting for past behavior, Nintendo is unlikely to have this game ready for launch since so far we've seen 6 seconds of footage and we first saw BOTW in 2014. I of course do not think we'll have to wait 2 or 3 years for the next Mario, just that they typically don't launch a game so soon after it is first shown, right?
Edit:
I can't tell for sure if you agree with me on this. You're saying BOTW would be better to launch with around the same time as Skyrim so Skyrim can appeal more to early adopters by being similar to BOTW? Not the point I was making (that Nintendo is better served launching BOTW with the system and eating up potential Skyrim sales than delaying it past the launch), but you bring up a good point.
The main point I'm trying to make is that I'm not sure why we should be believing any rumors that BOTW will not launch with the system. It seems much further along, perhaps only because we have seen so much more of it, and Nintendo has notoriously already delayed Zelda. (Twice, right?)