For faithful, literal bible/creation believers.
So assume the situation where God creates the world is true. His great creation becomes rather tainted due to that whole nasty business with Adam & Eve and the forbidden fruit. If God is the all-powerful creator, it would make sense to scrap these flawed humans and create new ones that won't be so likely to stray from his way. It was only a days work wasn't it? Whats a days work to the Almighty? So, why let an obviously flawed creation continue to grow and run wild on the precious world He created when he has the power to do something about it? Why not nip it in the bud?
He tried to correct things with the flood I guess, but ultimately that didn't seem to do a lot of good. I'd call that a situation of closing the gate after the horse has already bolted.
™ Version 4.2