Why did God flood the earth?
Race-mixing. [High Res]

Almost immediately after the pinnacle of God's creation, Man; the firstborn, Cain would take a wife from the humanoid beasts [Gen. 4:17]. Not only was Cain was in exile, having no access to Adamic women, this wife was prior to the birth of any Adamic women [Gen. 5:4].

Both populations would grow and become large enough to encroach on each others territory, inevitably leading to mixing. The mixed would grow and continue adulterating the pure, perpetually.

This adulteration would reach catastrophic levels, that God was needed to step in [Gen. 6]. God would save Noah, the only man left who was both pure in ancestry and faith [Gen. 6:4]. It is important to know he was saved for being pure in both ancestry and faith, as many others would be pure in ancestry only, such as Noah's sons and their wives; and many other Adamites.

The purpose of the flood was to protect and isolate Noah and his family. Although many Cainites and Adamites died, many would survive.