Why did the KJV use "thou" toward God?

In most languages an informal pronoun is used to address God. In German they use the informal 'du', for example. It's meant to signify a personal relationship with God, as in the way you would talk to your father.

To sum up what I was suspecting and what is suggested in one of the comments and (I believe) in one of the answers, God is referred to as thou (and its related case forms) because that is the only 2nd person singular pronoun used in KJV. The only possible case where a single person is referred to in the second person is Titus 2:7–8. The other epistle addressed to a single person (Philemon) uses only thou. All the dialogue involving Jesus or the apostles appears to use thou regardless of who is being addressed.

I don't know if we have any actual bible scholars on the site, but it'd be nice to have their opinion.