Who knows? I certainly don’t. The locution that only God knows the answer to something dates from the eighteenth century or earlier. (It is also put as God knows and Heaven knows.) Defoe wrote (The True-Born Englishman, 1701), “Great families of yesterday we show, and lords, whose parents were the Lord knows who,” and a few decades later Swift wrote (Polite Conversation, 1738), “Lord of the Lord knows what.”