Basically, they're seeing new possibilities for the game, and want to implement them.
Getting the 2015 deadline is no longer a priority, although it's not stated the game will miss that deadline for sure. Note that there are no problems with development, nor is the game in development hell.
The game will also not be shown at this E3 as a result.
Now, the internet obviously already reacted with their classic doom thoughts and such, but just my two cents, a perfectly acceptable move.
Not only will it probably not hurt the Wii U like so many people think (remember when SM3DW or Smash or MK8 were supposed to boost the Wii U significantly?), but if it makes the game better (and I'm pretty anxious about an open-world Zelda like this) I'll take the extra few months wait.