The project was weird from the reveal. The fact that Platinum Games developed this was confirmed after the Digital Event, not during (not even a logo). Why would you not advertise that the world's current best maker of action game, whose director has shown interest in the franchise, was indeed developing?
I have a feeling that Nintendo internally developed Star Fox first, realized the demand and of a "Platinum-made Star Fox" midway and contracted them to do grunt work instead of design. If we look at the latest trailers, nothing indicate that it was a Platinum game---and Platinum were know for their very particular brand of actions. Then came the delays and "development hell" comment.
This is a baseless observation from simply trying to connect why a game co-developed by Platinum does not share its DNA at all.
It's already been known for a long time that Nintendo developed a tech demo for Star Fox and sat on it since the Wii, only for them to resuscitate it for the sake of the Wii U and eventually try securing a secondary team that could weave a full game out of it - hence the way it was displayed in 2014. Much of the game play and mechanics is Nintendo while the design in the full game is mostly Platinum's work.