Historically, sultan is just a noble title (or more often a name). The later version of sultan meant the governer of a province under a caliphate (caliphate being a religious grouping, with the caliph as a very high religious leader, similar to a pope). The ottoman sultans fit this definition, since in the 1300's they claimed a caliphate as well (the ottoman sultans were separate from ottoman caliphs).

Since the Ottoman Empire ended, though, there hasn't been a widely recognized caliphate, and so Saudi Arabia is not a province under one. That means the Saudi nation is fully independent, so Ibn Saud became a king.

equivalent question (I think): Why is the emperor of Japan called emperor and not Shogun ?