Better: "Why is anyone not deserving of their just punishment?" :AMR:
The OP question presumes an obligation upon God without warrant.
God hath (a) Decreed in himself from all Eternity, by the most wise and holy Counsel of his own will, freely and unchangeably, all things whatsoever comes to passe; yet so as thereby is God neither the author of sin, (b) nor hath fellowship with any therein, nor is violence offered to the will of the Creature, nor yet is the liberty, or contingency of second causes taken away, but rather (c) established, in which appears his wisdom in disposing all things, and power, and faithfulness (d) in accomplishing his Decree.
We all, in our humanistic finitude, like to think that universalism is the answer, as it often makes our hearts ache to think that many persons will spend eternity in Hell. Sadly, too many of the faithful are not stirred up by this, preferring to sit around in church singing about Mama's favorite rocking chair, full of spiritual stagnation.
The simple fact is that God does not tell us why he saves a multitude that "cannot be numbered" and leaves the remainder in their sin. That said, God does admonish us to not even beg the question. Yet, we continue to disobey, talking back to God.
Underlying the OP is the modern era question involving the best possible world. Why did God create a world where some go to Hell and others do not? Hence, God is somehow not all powerful, nor all-loving. Sigh.
The Arminian and all its theological cousins have no answer, faced with the fact that so many won't even have a chance to exercise their so-called free will.
The Reformed answer may seem cold, but it is one with warrant. God is the potter, we are the clay. Attempts to bridge the Creator-created distinction with humanistic sentimentalities is a fool's errand. We do not know what is the wisest thing for God to do, but, God does. We should embrace and confidently trust in the wisdom of God versus attempting to ascend to the heights of God's abode. That has been tried once, and look what happened.
God provides an answer in the Cross, and we don't see these sort of questions being asked by the faithful about the supposed injustice of punishing an innocent man on that stick of wood. In fact many wear nice silvery crosses around their necks, so they obviously get it. (I do wonder why they do not also wear electric chairs for earrings.) But why don't they equally get it that if God can will the Cross, He cannot will other things that are equally disconcerting to our finite sensibilities?
Christ demonstrated the love of God towards His people, so much so as to ask that they be forgiven for what they were doing. But, for those that dishonored God, Christ showed no mercy. Not all are saved, else mercy is meaningless and justice is deniable.
AMR