Because the gates of hell have ALWAYS been struggling mightily against it. It has ALWAYS been filled with worldly or misguided bishops (Starting with the treacherous Judas and the cowardly St Peter himself, along with the others who abandoned Christ. And need I mention John XII and Alexander VI?)
Why did God let so many of his early followers be persecuted by the Romans?
Why did he let so many fall victim to the Arian heresy, or the Orthodox schism, or the Protestant Reformation?
Why let so many atrocities be done to the Christians during the Crusades, and to Catholic clergy in particular during the French Revolution and Napoleon’s period in power?
For that matter, why did he let his chosen people of old, the Israelites, be taken into slavery, first in Egypt - for 400 years, then in Babylon? And suffer so much else besides?
God does things in his own time, but he has also given us free will and lets us suffer the consequences of our own pride and stubborn sinfulness, but his promises are sure, as is his victory.