That's because Fathers tend to dictate the identity of the household. Pretty much every half-somali person I've seen completely rejects their somali side when they have an ajanbi father.
Don't lie. I've never seen a half Somali with an ajnabi mum speak their language or even properly identify with their Somali side.
In theory, you're Somali if your dad is, however, because language and culture is passed down through mothers, those said 'Somali' kids tend to feel closer to their mothers heritage, especially if the mother is from a strong culture.