[Elsie]

What’s the long answer?

[Campion]

It generally revolves around ideas like (a) "Not permitting it would be worse / have undesirable side effects" (cf. Parable of the Wheat and the Tares) or (b) "It's permitted for didactic reasons", e.g., to teach us something or test our faith.

It's probably not something we can completely understand in a satisfying way on this side of heaven, which is why theologians refer to it as "the mystery of iniquity". It's more-or-less a specific case of the general question of "Why does God permit the innocent to suffer?", which Thomas Aquinas said was the only objection to God's existence that had any substance to it.