Why does Zeus save Deucalion?

Zeus saves Deucalion because he recognizes Deucalion's piety and righteousness amidst a world filled with corruption and wickedness. As the Greek myth goes, Zeus decides to flood the earth to cleanse it of its moral decay, but he spares Deucalion and his wife, Pyrrha, to ensure the survival of humanity. Their survival serves as a means to repopulate the earth and restore a more virtuous society after the floodwaters recede.