I didn't say you need to have a conventionally attractive voice, just one that stands out.
Coco's voice definitely stands out when she's speaking Japanese, and the same goes for Towa and Ange. Mito is a bit of a special case in that she's one of Niji's pioneers and there wasn't exactly a lot of competition back then. If she were to debut today, I'm not sure she would get to where she is now.
I don't think it's a coincidence that all of the top holos (Pekora, Marine, Korone, Coco, Botan, Shark, etc.) have a fairly unique voice and tons of charisma.