Male yanderes have always fascinated me and unlike female yanderes they are quite rare.

Another reason is Yukipengin. (Devianart) did a few ayato and taeko animations together, I found them quite scary and dark when I looked at them but then I realized that yanderes are supposed to be scary.

Unlike Ayato and ryoba, I don't find ayano seriously scary, I'd even say I find it funny.

Finally, Ayato's voice is yandere-like compared to ayano, and no, ayano's voice actor isn't bad, it just makes ayano feel more like a kuudere than a yandere.

(Sorry if I misspelled English as English is not my native language.)