Because, although they learned to know God, yet they did not offer him as God either praise or thanksgiving. Their speculations about him proved futile, and their undiscerning minds were darkened.
The rest grew callous; as Scripture says--'God has given them a deadness of mind--eyes that are not to see and ears that are not to hear--and it is so to this very day.'
With their powers of discernment darkened, cut off from the Life of God, owing to the ignorance that prevails among them and to the hardness of their hearts.
Remember, God did not spare angels when they sinned, but sent them down to Tartarus, and committed them to caverns of darkness, to be kept under guard for judgment.
and that even those angels, who did not keep to their appointed spheres, but left their proper homes, have been kept by him for the judgment of the Great Day in everlasting chains and black darkness.