In Lord of the Rings why was the Throne of Gondor vacant for so long?

Answer 1

During the Battle of Fornost, Eärnil II's son Eärnur led Gondor's forces to victory over the Witch-King of Angmar, who later became the Lord of the Nazgul. Although Eärnur wished to fight him, Eärnur's horse was terrified and fled the battle against his wishes. By the time he mastered his horse and return, the Witch-king had fled. Glorfindel the Elf then prophesied to him that it was better that he not fight the Lord of the Nazgûl because not by the hand of man will he fall. Eärnur later ascended to the throne, ruling from Minas Anor (Tower of the Sun). During this time, the Ringwraiths captured Minas Anor's twin sister city, Minas Ithil (Tower of the Moon), renaming it Minas Morgul (Tower of Black Sorcery) and taking it as their lair. Minas Anor was renamed Minas Tirith (Tower of Guard) as a result. The Lord of the Nazgûl repeatedly sent messengers to Minas Tirith challenging Eärnur to single combat, taunting him that he had fled out of cowardice from facing him during the Battle of Fornost. Eventually, King Eärnur was overcome by wrath and rode with a small company of knights to Minas Morgul, to accept the challenge. They were never heard from again. Due to suspicions by the Kings of Gondor in the wake of the Kin-strife, many men from the Royal House had either foresworn their heritage and taken wives of non-Númenórean blood, or else had fled into exile. Upon Eärnur's departure, no one could be found who had an equal or stronger claim to the throne than would have any son of Eärnur (who left no children). The rule of Gondor was left to the Stewards of Gondor because there were fears of a civil war if a person who did not have the support of most Dúnedain of Gondor was crowned King. The weakened southern kingdom knew that if a civil war had happened, its many enemies would swoop in and destroy it in a heartbeat. The realm was governed by a long line of hereditary Stewards after the disappearance of Earnur, son of Eärnil, since there was no proof that the last king was dead, and no claimant had enough support to be accepted as his successor, and in the custom of Gondor the stewards would govern the land in place of the King should he be absent from the throne. The Line of Anárion was held to have failed, and Gondor was not willing to risk to another Kin-Strife, which would surely have destroyed the realm. Each succeeding Ruling Steward thus swore an oath to yield rule of Gondor back to the King, if he should ever return, but as the generations passed into centuries, the oath became (in the eyes of the Stewards) more a formality than anything else. But eventually the heir returned as Aragorn son of Arathorn, though he was indeed from the line of Isildur and not Anarion.