The reason is simple: technicality.
Manga designed to be black and white on slightly white gray paper that turned yellowish overtime.
On old school manual method, to paint pitch black they use ink, but for rendering or coloring shadow they use thin pen line, for something in beetween black or white,they use a tone that need to be cutted and then stick to the page manually, which is painfully long and taking alot of time, not to mention its hard to make detail in dark color to pop of. So most mangaka avoid it or atleast not make a main character that need to use it often.
In modern digital way, its take less time and more easier to apply or setting the color tone. But oda artstyle already fixed to only use black and white like many other mangaka in their starting era. And no, recolor an already established charachter is a no go
So yeah, sorry to burst the racism buble,the answer is just technicality.
here some example:
Yoruichi first appearance(manual): notice how murky the shadow on her body look?
And this when the digiral era come:
Berserk casca dark skin not even colored tbw:
Black beard first appearance:
Even afro samurai:
clownass premise
