>>211685552
It's because the most famous onryo story, the Yotsuya Kaidan, is about Oiwa, so her likeness is synonymous with the genre. That's why not only are all onryo female, but they all look exactly the same (long black hair, pale skin, white dress). It's the same reason when we think of vampires, we immediately think of a well-dressed Eastern European guy in a dark cloak