Why can Afar and Somali speakers not necessarily understand each other?

Although the Somali and Afar languages are both members of the Cushitic languages family, they are not mutually intelligible.12

While they share a common linguistic ancestry within the Afro-Asiatic phylum, they belong to distinct subdivisions of the Cushitic family. Somali is classified within the Omo-Tana group, whereas Afar (along with Saho) is categorized under the Lowland East Cushitic subdivision. Because these languages evolved separately into distinct systems with their own specific grammatical structures, phonological rules, and vocabularies, speakers of one cannot automatically understand the other.31