Why did Elohim destroy all but eight by water and subsequently focus primarily upon Noah and Shem?

The focus of Scripture, its purpose, is to reveal Elohim’s eschatological plan, His battle strategy, to restore peace and order to His Kingdom that was disrupted before Time by a rebellion initiated by God’s anointed cherub angel we know as the Devil, Lucifer-Satan-the Evil One-Serpent. From the Genesis (Genesis 3:15;21) to the Revelation, Elohim’s focus has been upon His battle strategy, the Gospel ordained before Time began (2 Timothy 1:9-10), through which He would “destroy the works of the Devil” (1 John 3:8b); unfortunately, the Adamic/Noahic generations became so defiled (Genesis 6) that it was not possible for Elohim to set-apart a people group after His own Name through whom He would introduce His Warrior, His Messiah, into the Realm of Time; therefore, Elohim destroyed all of mankind save eight and focused upon Noah and Shem and Arphaxad through whom came Abraham, Isaac, Jacob and the Twelve Tribes of Israel through whom Judah was to be the deliverer of Messiah in Time.