Something they probably just never considered. We do see on occasion the mouth color be different colors so it's hard to say why this occurs other than it just does. Just not a lot of consideration into why they look the way they do other than they just do.
My headcannon for the weirder traits like this is that the Amengi modifications had something to do with it. Like maybe it made locating a fugitive Predator easier to find if the blood glowed? Though it could be that it doesn't glow when in the body, maybe something happens to it when it reacts to air? There's also that weird bit of Neca lore where Stalker comes from a dark area of the homeworld and his skin is so pale parts of him glows. I feel like Voodoo should know the lore reason somehow.
