But they change their ground and endeavour to vilify our faith in another way. They ask, if what took place was not to the dishonour of God or unworthy of Him, why did He delay the benefit so long? Why, since evil was in the beginning, did He not cut off its further progress?— To this we have a concise answer; viz. that this delay in conferring the benefit was owing to wisdom and a provident regard for that which would be a gain for our nature. In diseases, for instance, of the body, when some corrupt humour spreads unseen beneath the pores, before all the unhealthy secretion has been detected on the skin, they who treat diseases by the rules of art do not use such medicines as would harden the flesh, but they wait till all that lurks within comes out upon the surface, and then, with the disease unmasked, apply their remedies.
When once, then, the disease of evil had fixed itself in the nature of mankind, He, the universal Healer, waited for the time when no form of wickedness was left still hidden in that nature. For this reason it was that He did not produce his healing for man's disease immediately on Cain's hatred and murder of his brother; for the wickedness of those who were destroyed in the days of Noah had not yet burst into a flame, nor had that terrible disease of Sodomite lawlessness been displayed, nor the Egyptians' war against God, nor the pride of Assyria, nor the Jews' bloody persecution of God's saints, nor Herod's cruel murder of the children, nor whatever else is recorded, or if unrecorded was done in the generations that followed, the root of evil budding forth in various manners in the wilful purposes of man.
When, then, wickedness had reached its utmost height, and there was no form of wickedness which men had not dared to do, to the end that the healing remedy might pervade the whole of the diseased system, He, accordingly, ministers to the disease; not at its beginning, but when it had been completely developed.
Source: The Great Catechism (New Advent)