Good heavens, indeed!
Shigeru Miyamoto, the series creator says that the physics enging being built from scratch played a huge role in the delays. The developers would prefer if they 'didn't have to put a release date out at all,' but that's not really an option for game makers these days.
The Legend of Zelda: Breath of the Wild was originally slated for a 2015 release, but that was then pushed back to April 2016, and now it's set for 2017. Check out the trailer below: