Short answer: Don't ask this question. God is God and we are but mere men. He is not obligated to explain Himself to us.
Slightly longer answer: Job 38-41.
Lame, unbiblical answer by people who need an explanation for everything: God gave us freedom. If he restrained evil, we wouldn't be truly free.
On a related note,
Rom 9:18 So then he has mercy on whomever he wills, and he hardens whomever he wills.
Rom 9:19 You will say to me then, "Why does he still find fault? For who can resist his will?"
Rom 9:20 But who are you, O man, to answer back to God? Will what is molded say to its molder, "Why have you made me like this?"