According to this Forbes article, Super Mario Galaxy Movie producer Chris Meledandri spoke about how the movie came together.

In regard to Star Fox, Chris had asked Miyamoto about adding Fox to the movie and Miyamoto liked the idea, but thought there might be push-back internally from Nintendo, saying they don't usually like to mix IPs. However, there wasn't much push-back at all and Miyamoto got approval to consider a role for Fox in the Mario movie.

Meanwhile, the pair got called up by actors from across the industry after the first Mario movie launched, seeking to be part of the Nintendo universe of movies.

In particular, Glen Powell phoned them went on a passionate explanation of how important Fox McCloud was to him. He dreamed of being part of a Star Fox movie, if that ever happened. Neither Glen nor anyone else knew that Miyamoto and Meledandri were considering adding Fox to the next movie.

Glen's conviction and passion was so strong that they settled on adding Fox. They called Glen back and explained that Fox would be in the next Mario movie and he had to keep this a complete secret.

And the rest is history. We all got a surprise announcement about Fox just a few days before the film's release. Glen's Fox McCloud played an important role in the movie, and his voice and enthusiasm matched well with the character.

I speak for a LOT of Star Fox fans when I say, "Thank you, Glen!"

Also, we too hope there is a Star Fox movie one day. Actually, there's enough story for at least a trilogy. Fingers crossed Nintendo gives Star Fox (and Glen) that chance soon.