Its a great question. Its a question that gives an answer in itself really. Some do and some do not.
But, as we can see the real question is "why". Why indeed...
I mean, its a great proposition He has given us. Freedom from slavery to sin, freedom from the eternal consequences, promises of eternal life, promises of a close loving relationship, promises of unspeakable things that are too grand to even imagine.
All of this, and yet some come to Him, and some do not.
We could argue the fact that some are "chosen" to come and some are "chosen" to not come. Predestination/5 point Calvinism
Then there is the line of debate that says all will come at some point in time. Purgatory.
But I think that the best one to debate, the best one that addresses this situation, is freewill.
Man has been given a choice by God to accept His gift or not. And people debate the point of freewill, but loose out on the meaning behind it, which is a God who wants to show His power and love on a "free" basis.
He is a God who wants to be known as real, and even when the deepest darkest sin reveals its ugly head in the life of a believer, because that believer has freewill, and yet God chooses to forgive them, love them, teach them, welcome them......
Some come, and some do not. Some come because they see the Creator who loves them, and want to love Him back. Some do not come because they see the creation and want to love it.
The answer to the question can indeed be known for sure half way. Some come because they hear the call of God, because without the call no one can come. But even in that answer to half of the question we get a glimpse of the answer to the other side of the question.
God calls on all men. And God is God, so therefore if He calls we will hear. So the reason why some do not come is because they choose to not come to the call.
The Bible tells us that God is long suffering, not willing that any would perish, but that all would come to repentance. Coming to repentance is where the line is. Some draw the line at the call of God, which only serves to mask and camouflage the real line. It is trickery of the one whom is on the other side of the line.
People do not come because they do not want to turn to God. So, I think it moves us into another question and answer time of;
Why do some not want to turn away from sin and accept the free gift of God?