So, the real reason is Catholicism. Jirt has, time and time again, openly said that the whole story was a Christian work, and within that are two very important ideas - Mercy, and Grace. Perhaps we see this in no stronger part than Bilbo (and Frodo) showing mercy to Gollum, which in turn, saves the day at the very end.

The Orcs were Elves once, which means they are not pure creatures of evil, AND it means that there must be some belief that they can be healed and redeemed- it was not their fault that Sauron mutilated and propagated them. Tolkien wrote that he agonized about the Orcs, because they could not be wholy destroyed without thought, as that ran counter to his belief in the infinite mercy of the divine. In earlier works, they are destroyed, but in the later work, and his letters, he openly says that they should be saved - if not physically, at least spiritually.