I don't think they're wedded to the Sword Coast or even the Realms for a movie. Their target audience for a big budget Hollywood movie isn't primarily D&D players, its the public that likes big budget action fantasy movies, that has no idea what the "Forgotten Realms" are. They know that no matter what setting it takes place in, D&D players will see it, or at least care more about the quality of the film than where it is set (except for the most unreasonable anti-Realmsians, who are probably a small minority of keyboard warriors).
Their focus will be, and should be, to create a great fantasy movie and franchise that uses the great wealth of D&D material (monsters, worlds, stories, etc).
As for why WotC likes the Sword Coast, if we try to look at it without bias, it is a pretty complete region that is great for D&D.