Comment by erutuon

Good insight that I hadn't thought of. To flesh that out, the KJV translators used thou as singular and ye as plural consistently to render the second person singular-plural distinctions found in Hebrew, Aramaic, and Greek. The translators never intended a usage of thou in the KJV to imply informality, only that the original language used the second person singular. That would naturally carry over into prayers and hymns that don't come from the KJV Bible. However, common people at the time who couldn't know the translators' chosen convention might have then read closeness into the pronoun used to address God, which wouldn't be a bad thing theologically.