It's specifically an issue about making PoC characters white. When a character is apparently not Caucasian, making them so can be seen as a bit…racially insensitive. This is mostly in context with America's history with racism, but it can be applied to similar scenarios in other parts of the world.
On the other hand, changing sex/gender is often seen as either exploring their sex appeal or simply seeing what they look like with a more masculine form (which, unlike skin tone, actually affects a character's whole silhouette and can thus more drastically alter the mood of their design).
At least, that's my understanding of the situation (which could be flawed, so anyone feel free to correct me if i'm wrong)
Personally, I just think it's unnecessary to change a character's skin color. It doesn't really do anything except spark controversy on the internet. I suppose, then, that some people might do so to exploit the free exposure, but that's probably not always the case.