Charles Ellicott Commentary 1 Kings 2:27

"So Solomon thrust out Abiathar from being priest unto Jehovah, that he might fulfil the word of Jehovah, which he spake concerning the house of Eli in Shiloh." — 1 Kings 2:27 (ASV)

That he might fulfil.—In these words is described, not the purpose, but the effect of Solomon’s action. The prophecy referred to is, of course, that of 1 Samuel 2:30–35; 1 Samuel 3:11–14, fulfilled by the degradation, in Abiathar’s person, of the house of Ithamar, and the exaltation, or restoration, in Zadok, of the house of Eleazar, to whom, as the elder son of Aaron, the primacy would have seemed naturally to belong.

It seems clear from 1 Kings 2:35 that Abiathar had, until then, held some superiority, although in the various mentions of the two, Zadok’s name stands first; but whether of actual authority, or only of priority of dignity, cannot be determined. While the Tabernacle remained at Gibeon under Zadok’s charge, and the Ark was in Mount Zion under Abiathar, there might, indeed, have been something like co-ordination between the two. This, in any case, must have disappeared at the building of the Temple; and the disgrace of Abiathar determined that the undivided dignity should pass to Zadok.