Why on (Middle-)earth was Isildur allowed to keep the One Ring?

Long ago, Sauron wreaked havoc upon Middle-earth. After Celebrimbor first created his rings of power, Sauron forged his own - intent on domination. He took Celebrimbor's rings and used them to seduce nine kings of men to his side and to attempt to corrupt seven dwarven lords. Laying waste to Eregion, he pursued Elrond, who established Rivendell as a stronghold and shelter.

His war of domination was only thwarted by the power of Numenor. [More]