Well, it's an honest answer, and a true one. There's nothing wrong with saying "I don't know".
Pope Francis Says He Doesn’t Know Why God Lets Children Suffer
Of course, as an Existentialist, I'd say the answer is "Because we live in a world governed by chance and scientific laws which are indifferent to human lives", but I don't have this guy's job.
Best Pope in a long, long time.