In a post on the official Nintendo Facebook page, Nintendo producer Shigeru Miyamoto clarified the delay:
We have been developing Star Fox Zero for Wii U with the aim of releasing it this year. Although we felt that the development had been progressing well, we now believe that we will need a little more time to work on areas such as the unprecedented discovery that we want players to experience in the game by using two screens, and further polishing the level designs and perfecting the tone of the cut scenes. While we have already reached the stage where it would be technically possible to release the title in time for the year-end holiday season, we want to polish the game a bit more so that players will be able to more smoothly grasp the new style of play that we are proposing.
To the people looking forward to the launch of the game this holiday season, I am very sorry.
Star Fox Zero is going to bring new game play and experiences that take it far beyond the framework established by Star Fox 64. All the members of the development team are doing our best so that the final product will not betray your expectations. And the game will not be delayed for a very long time – we’re aiming to launch the game in Q1 2016. Please stay tuned for further announcements.
The latest trailer for Star Fox Zero was released at E3 in June.