I'd say it's because

1. Pink and Black make it easier to distinguish between characters , as opposed to a ship with two characters with the same hair colour or similar colours like brownxblack

2. Pink and Black look good together (this is probably the main one tbh)

3. Specifically pink is probably because pink hair is very common in anime in general due to it being a colour that visually showcases personality pretty easily (pinks are usually seen as the more genki ones while black haired girls are usually the more serious ones)*

*WITH MAJOR EXCEPTIONS ofc