Gondor history explains the transition from kings to Stewards

The history of Gondor's leadership prior to the events of ‘The Lord of the Rings’ explains why the realm was governed by Stewards rather than kings. The royal lineage, which originated in the island nation of Númenor, established Gondor and Arnor following the destruction of their original home.

Following a civil war that decimated the royal family, the monarchy became increasingly insular, leading many claimants to flee or renounce their rights. The line of kings eventually faltered during a conflict involving the Witch-king. Prince Eärnur, the heir to the throne, was sent to assist Arnor, but a confrontation with the Witch-king caused his horse to bolt, an event that significantly impacted the transition of power and the subsequent era of the Stewards.