I'd guess it's just a matter of desire to add it. Italics were added for v7 using ctrl+I (control code 29) - plenty of other control codes left that, to other clients, would just appear as an odd error until they would add support for it. And therein lays the rub. Think about a decade back and you might have seen oddball messages ("# appears as NAME NAME") from Microsoft's chat client - nothing else embraced that (avatars, I suppose) and those users were often finding themselves kicked swiftly.
If more colors did end up being allowed, I would imagine it would actually have an RGB type syntax. mIRC would probably have to drop its palette-based drawing, though.
