There are multiple possibilities :
1 - I rule out the clear racism especially knowing what happens in the manga, but a simple lack of the understanding of the importance of representation might explain that and a bit of racist bias in that. (I would go for that first)
2 - It's not easy to represent skin tones in black and white. In reality, you have not a lot of choices and you are forced to used trames to fill dark spaces (I don't know the english equivalent). So it can be time consuming which could explain why some avoid it. But I don't think that is the case for Oda who has assistants.
3 - Some mangaka might have thought - wrongly - at first that the design was enough to suggest the skin of the color. This is a problem of representation once again
4 - Like I said, for black characters, you are forced to used trames. Meaning that it become harder to cast shadows on their skin. It can explain why some might be reticent to depict them. Even tho it's not really a good excuse.
5- (what I would go for that second). It's by design. If you look at the manga before wano, characters who have darker skin are not really that dark, it's more in the nuances and they only appear that way through the coloring. But black characters with darker sking appears with King and Lunarians.
A possible hypothesis might be that Oda intended to introduce the disappearance of the Lunarian race at Wano.. but the story took more time that expected to arrive to this point.
Also.. outside of the black and white : a good coloring makes the difference:
(Yes, go read Radiant)