If God kept Mary from Original Sin, why doesn't He keep others from it?
If Mary could be kept from Original Sin, let's say somehow thanks to what would be Christ's future sacrifice, why doesn't He do the same for others? Is something preventing Him from working the same grace? Sure, Mary bore Jesus, but why would that be a requirement in the first place?
Answer
God will do the same for others, when they get to the New Heaven and New Earth, and it will happen because we will be like God. But God is only going to give that power to those who have proven to him, on Earth, that they love him; those who have spent their whole lives proving to him that they want to live with him for all eternity in holiness; those who have sacrificed everything else for the pearl of great price, as it were. And in order to make that possible, God had to keep Mary from Original Sin, otherwise we would have no Savior to make the above possible.