Big robots fighting may not be as popular domestically as abroad, but Gundam has more cultural cachet than something like Pacific Rim and with Brian K. Vaughan writing, it might actually be good.
Mobile Suit Gundam first premiered 40 years ago and in the decades since it has spawned many anime series, movies, manga, novels and toys (merchandising=$$$).
The franchise has generated billions over the decades, and because there is no single Gundam story, Legendary’s film can still bear the name and be an entirely new narrative without risking claims of not being faithful.