From the Catechism para. 412:

Question: But why did God not prevent the first man from sinning?

Answer: St. Leo the Great responds, "Christ's inexpressible grace gave us blessings better than those the demon's envy had taken away". And St. Thomas Aquinas wrote, "There is nothing to prevent human nature's being raised up to something greater, even after sin; God permits evil in order to draw forth some greater good." Thus St. Paul says, "Where sin increased, grace abounded all the more"; and the Exsultet sings, "O happy fault... which gained for us so great a Redeemer!'' (See also Rom 5:20).