There's a difference a family movie for children 12 and up (which may interest kids that around 6 or 7, but lose interest for kids that are 3-4) and a family movie for children 3 and up. From Nintendo that's as they can cross promote merch and video games.

When you are making a faithful adaption of a game about a guy going through levels rescuing a damsel in distress. There's not a lot to say. Too much extrapolation may be unfaithful to the game.

Safe is best.

Example on expanding on the source material:

Zack Snyder Man Of Steel. It made money, but it divided the fan base. Not good for IP and to build upon. Movies after that further alienated the base.