You have raised a good point.
We need to know that it is not you and me who say that criminals guilty of polytheism will not be forgiven. It is the Lord who declares in the Holy Qur'an that He will forgive every sin but not the filthy act of polytheism.
The reason seems to me is the fact that the Lord is one and alone is patently obvious to mankind. It is indeed a built in belief and their intuition bears testimony to the Oneness of His Lordship. The Holy Qur'an has one awe inspiring anecdote to relate about mankind in this regard. It says that Allah, before sending any human on earth, had all the souls made one pact with Him. The Lord asked them if he was their Lord and they all replied in His very presence. Why not! Could that stark reality be doubted!? It seems that all humans are carrying the remembrance of this incidence within their souls. This is not tangible, I accept. But like the existence of conscience is not tangible yet we can tell by experience that something like conscience exists, we can also feel the influence of an inward assertion that there is One God. It is for this reason that the majority of mankind has always believed in One God. Although there are polytheistic practices and beliefs, the majority conclusively arrives at one end: there is one Supreme Being. In summary, this belief in one God is inherent. The following verses of the Holy Qur'an refer to this:
And (remember) when thy Lord brought forth from the Children of Adam, from their reins, their souls, and made them testify of themselves, (saying): Am I not your Lord? They said: Yea, verily. We testify. (That was) lest ye should say at the Day of Resurrection: Lo! of this we were unaware. (7:172)
There is however one thing to remember that Allah shall be the Judge on the Last Day of Judgment. He is absolutely just and will definitely dispense the justice in the perfect manner. Therefore, anyone who will come up with a reasonable excuse for his crime will be given allowances by the Lord. Actually, we do not know what is in the hearts of men, but Allah does. He therefore takes into account each and every point that will merit consideration before He will finally decide the matter.
Why God will not forgive polytheism when man can forgive every crime perpetrated against him? I have reason to doubt this. Every person who has sense of honor and self esteem cannot forgive a crime that is committed against his honor though he can forgive crimes against his wealth and even life but never HONOR. Honor is not be compromised. No crime against honor will be forgiven unless some genuine excuse is presented. This is so true about Allah the Almighty. Allah does not want, under any circumstances, that anyone should dare associate with Him anything that is not like Him.