Why does God forgive all sins except for Blasphemy of the holy spirit?

Answer:

Blasphemy against the Holy Spirit is "quitting on God"... and God hates quitters.

The ones whom God gives His Holy Spirit are those who have repented of their sins and accepted on faith the power of Jesus' shed blood to wipe all of their past sins clean... blotted them all out... having the death they earned for their past sins paid in full by God, Himself.

They were baptized in His death... resurrected from their watery grave, washed clean of all sin... morally cleansed and ready to begin life anew in obedience to His voice and His laws. Living according to His Word.

Then, they will be given the "earnest" of God's Holy Spirit to feed them the Truth of His Word, the Bread of Life, for the rest of their lives (or until Jesus returns, whichever comes first). And they become a growing, developing child of God, if they are nourished by "God's Bread" and don't stop feeding on it.

"...Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Spirit." (Acts 2:38)

Being born is a struggle... for the mother and the child. And so is the life of God's Children who are developing in Godly character. It is a radical change from the nature of the flesh to adhere to the commands of the Spirit (Rom.8:7). It goes against the grain of human nature to obey God's laws:

"...for we know that the law is spiritual: but I am carnal, sold under sin... for I delight in the law of God after the inward man: but I see another law in my members, warring against the law of my mind, and bringing me into captivity to the law which is in my members... I thank God through Jesus Christ our Lord. So then with the mind I myself serve the law of God; but with the flesh the law of sin." (Rom.7:14-25)

Being born is a struggle. Being reborn is a spiritual struggle that the child of God must
"OVERCOME" through faith in Christ. And the nourishing Word of God provided by the Holy Spirit in the child is the source whereby its strength and growth comes... and overcomes.

So, blasphemy against the Holy Spirit can only be committed by one who HAS THE HOLY SPIRIT DWELLING WITHIN THEM... and they QUIT ON THE STRUGGLE... and return to their nature of selfish and vain fleshly pursuits.

It's knowing the Truth of God... having Him entrust you with the Holy Spirit (a spark of His very eternal life force)... and throwing it back in His face. Thumbing your nose at Him.

It's quitting on God... denying faith in Him after having one's spiritual eyes and ears opened to His Truth. It's virtually self-abortion... or at the very least a "miscarriage."

"...for it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Spirit, and have tasted the Good Word of God, and the powers of the world to come, if they fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put Him to an open shame." (Heb.6:4-6)

Blasphemy against the Holy Spirit is having it dwelling within you, feeding you the Truth of God's Word, helping you overcome the flesh and the pulls of the sinful world... and you GIVING UP THE STRUGGLE.

It's the conscious descision to NOT BE REBORN. It's consciously denying oneself eternal life. It consigning oneself to eternal death.

"...but those mine enemies, which would not that I should reign over them, bring hither, and SLAY THEM BEFORE ME." (Luke 19:27)