They probably delayed it since they're going to reveal a new Metroid like Prime 4 for the NX at E3 and realize it's better to release Federation Force after a real Metroid game is revealed. This way all the pathetic Metroid manbabies will finally shut the **** up about Metroid being dead and Nintendo will have an easier time advertising Federation Force for the kind of spinoff title it is.
Plus this will help get the game better reviews since we all know most of the negative Federation Force reviews right now will just be about "Bawww, where's my real Metroid". By waiting until after E3, that way the game will automatically jump up a few points since reviewers might actually play the game now instead of trying to write worthless editorial pieces about something not related to the actual game they should be playing.