From what I understand, there are some in the Islamic world who hate that the House of Saud refers to themselves as king, because the only king is supposed to be Allah. So why didn't the Saudis take on a more traditional Islamic title such as Sultan? From my understanding, Sultan is a high king.
https://www.reddit.com/r/AskHistorians/comments/2tgy98/why_was_kingdom_chosen_as_the_term_for_the
Bit late, but this thread should help.
Particularly u/Labrydian 's response
Largely comes to prestige, Ibn saud taking the Arab title for king after combining the Sultanates on the peninsula