For the same reason that He allowed Gnosticism, Nestorianism, Arianism, and every other heresy - so that the Church would be moved to define its teachings even
more clearly, and bring us
even closer to a full understanding of Christ.
After all, if there had never been any heresies, there never would have been any Councils, and there are so many things that would be more difficult for us to understand, and to find out, because the definitions for them would not exist.
But iron sharpens iron, and fierce opposition forces us to clarify our teachings, and beak them down into small concepts, simple enough for everyone to understand them.
I have also noticed that competition improves the final product. A Catholic Church that is situated within walking distance of one or more Protestant churches tends to be a lot more orthodox in its teaching than one that has no such competition.