Why aren't all people saved? Some turn to Jesus, but why don't others? Some get it, but why do some not?
The Bible gives us multiple answers. Each of these answers are not contradictory but rather complimentary, giving light to a different facet of this complex issue.
1. Because men love darkness.
The reason why people don't turn to Jesus is because we love darkness (John 3:19). The responsibility falls on us. We are culpable for our own rejection of God and His Son.
2. Because no one is able to come to the Son except the Father who sent Him draws him.
Jesus peels back the curtain and declares more than once in the sixth chapter of John's Gospel that people lack the ability to come to Him (to believe in Him, to spiritually discern and accept Him and what He says) until and unless the Father who sent Him draws them (read John 6:22-71). If you consider the context, you will see that Jesus states this directly in response to grumbling of his audience. Christ's response said in effect, "Yes, I know you don't get it and can't accept it. It is because you're spiritually blind and the Father has not opened your eyes." He even explicitly says to His disciples, "This is why I told you that no one can come to me unless it is granted him by the Father."(v. 65)
3. Because God created from one lump of clay some for common use and some to display His mercy and grace.
After eight Gospel-saturated chapters in Paul's letter to the Romans, the apostle finally addresses the issue of why some -namely, why so many Jews, who were supposed to be God's chosen people- reject Christ and are lost. What happened? Were God's promises nullified somehow? No, and Paul set out to give us a behind-the-scenes view into the hidden purposes of God (read Romans 9).
Here, the "one lump" of clay represents all of sinful humanity. The Potter (God) justly decides some are to be molded for common use and some are to be molded into trophies of his grace and mercy. Since all are under sin, He does no violence nor injustice in choosing to leave some of us to our own way, as vessels fit for destruction, while choosing to have mercy and grace on others whom he has chosen to be vessels of mercy.