I'd argue that it's a general manga thing that ends up being especially relevant for BL stories like this with a harem's worth of dudes.
Cuz like, there are a lot of manga art styles where the facial structure, skin tones, builds, etc., are such that the main difference between a lot of the characters is hair and boob size figure, and when you use a style like that to draw BL you lose figure. Makes hair and hair color super important.