God has purposely blinded some for the good of the Elect. Nobody knows why God draws some souls to Himself while permitting others to remain in ignorance. Most agree that God's predestination and predilection is not based upon any merit, but God's pre-ordainment at the beginning of the world. As St.Thomas says, the reprobate "seem to be preordained by God for the good of the elect, in whose regard all things work together unto good"
When Jesus was rebuking the Jєωs, He explains: "No man can come to me, except the Father, who hath sent me, draw him; and I will raise him up in the last day”. This verse reveals that if one is to come to Christ, he must be drawn by the Father's efficacious grace; he cannot come to Christ by his own natural powers. Because obtaining eternal life with Christ surpasses our natural abilities, man must be directed (or predestined) to that end by God. From this follows that many are blinded and hardened by God, those whom Christ says that "have ears but hear not, that have eyes but see not".