I think it has to do with each persona's functional role. For example... The Father is the unknowable YHVH functioning as Father. The Son is YHVH functioning as redeemer. The Spirit is YHVH functioning as regenerator and sanctifier.
One can speak against God's role as a Father, much like a child making a temper tantrum and saying hateful things to their mommy or daddy. One can speak against God's role as redeemer, but the atonement is already made. But if one speaks against the Spirit... the Spirit is the functional persona of God that breathes on a man or woman's spirit and opens his or her eyes to truth. The Spirit is that functional persona of God that will sanctify a man and bring all truth to light. The Spirit is that functional persona of God whereby we are partaker's of God's very nature in that He abideth in us... and us in Him. So speaking against the Spirit essentially causes one to be hardened against truth, against sanctifying power, against the very divine nature one must partake in to be saved.
Notice...
Matthew 12:22-32
English Standard Version (ESV)
Blasphemy Against the Holy Spirit
22 Then a demon-oppressed man who was blind and mute was brought to him, and he healed him, so that the man spoke and saw. 23 And all the people were amazed, and said, "Can this be the Son of David?" 24But when the Pharisees heard it, they said, "It is only by Beelzebul, the prince of demons, that this man casts out demons." 25 Knowing their thoughts, he said to them, "Every kingdom divided against itself is laid waste, and no city or house divided against itself will stand. 26And if Satan casts out Satan, he is divided against himself. How then will his kingdom stand? 27And if I cast out demons by Beelzebul, by whom do your sons cast them out? Therefore they will be your judges. 28But if it is by the Spirit of God that I cast out demons, then the kingdom of God has come upon you. 29Or how can someone enter a strong man’s house and plunder his goods, unless he first binds the strong man? Then indeed he may plunder his house. 30 Whoever is not with me is against me, and whoever does not gather with me scatters. 31 Therefore I tell you, every sin and blasphemy will be forgiven people, but the blasphemy against the Spirit will not be forgiven. 32And whoever speaks a word against the Son of Man will be forgiven, but whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come.
The Pharisees attributed the power and work of the Holy Spirit of being "of the devil". That means they cut themselves off from the very agent of God's regenerating work, sealing their fate to remain alienated from God, never to be reborn by the Spirit.
Think of it like this...
A child can curse his daddy in a grocery store while throwing a temper tantrum, the father still loves the child, though he disciplines him.
A child can curse the fact that daddy goes to work to provide food, clothes, and shelter...but the work has already been done and the child is still under the roof and seated at a table with a meal before him.
But if there is a fire and daddy is trying to convince the child to jump from a second story window into his arms... if the child curses his father, refusing to jump to safety, he will perish.
The functional role of the Holy Spirit is to move upon the heart, bringing the human spirit to life to receive the things of God by faith. Curse that... and what hope has any man?