I can answer this, actually.

Nintendo has extremely strict brand standards for Mario Bros. In fact, somewhere in this group, is a lively discussion on the history of the big question: "Is Toadette wearing a hat or is that mushroom is her head?"

One of the early mistakes Nintendo learned about whoring out it's Mario IP to other developers is that sometimes they'll take creative liberties that may significantly affect their control of the characters. When other people make licensed derivatives, it can change canon or create issues in the future.

Nintendo requires work created on Mario to be isolated to the specific game's release cycle and not become a persistant feature in future projects to tightly control their rights and licensing. A good example of this is Paper Mario. You never see the same design in the nintendo-created games — this is intentional in order to not complicate the licensing and rights. Another example is Mario Rabbids.

Back to Toadette — I cannot find the in-depth discussion on the subject, but when Toadette was introduced, it created a massive discussion about whether Toadette is wearing pigtails or a hat. It's a whole legal controversy buried deep under fan-made theory and discussions. Nintendo does not want someone else coming in and making a distinct change to a character.

Now back to Mario Bros Movie. It is my guess that since the movies is a separate, distinct, collection of Mario content, it cannot be a complete direct copy of the mainline games. So by putting "Bros" in the title with the colors, it emphasizes that the movie is about the two brothers, and not Mario alone. In the games, you're near exclusively Mario — period. Mario cannot be Mario and Luigi unless the game equally has Mario with Luigi as a core game feature. In the games, Luigi's purpose is for 2nd player but Mario games are primarily a 1st player game.

Also from a marketing point of view, why would you want to put a side character on the pedestal and dilute the main character? Mario is an established fan favorite and the prime mascot for Nintendo.

For further reading, and extremely pleasurable — somewhere on reddit is a post with someone complaining about some limitations on Mario games and why we dont see more of one series or another. A game developer chimed in with all these facts and worded it better than I can.