This is a trope previously discussed in a recent post: Reverting Back to Yellowface. While it might be a script issue, sometimes it happens during casting. Even when the film specifically stars people of color, the Mighty Whitey still robs them of being the main characters, pushing them back as supporting characters to their own struggles, for what is typically a straight white man.
Basically, people of color can’t be trusted or believable in a film to save themselves, unless the Caucasian male arrives. Even the “historical accuracy” defense can’t be used because it is warping the reality, such as Stonewall (on top of eliminating Marsha P. Johnson), or myth it is base on like the previous Mulan rumors. Below is a slideshow that discusses this in the first half, but if you’re interested here is a list.
[Gif by Vanity Fair]