Why would that be unforgivable? Because it is precisely through the power of the Holy Spirit that we as individuals receive God's forgiveness, In fact, it is only by the power of the Holy Spirit that we even ask for that forgiveness!
Blasphemy against the Holy Spirit cuts off the very channel that makes forgiveness possible.
Now, it is rather unlikely that even a nominal Christian would ever commit formal blasphemy against the Holy Spirit with sufficient reflection and full consent of the will. Just to be safe, however, we should be careful to practice discernment, determining prayerfully what is of God and what is not, lest we follow the path of the Pharisees in this chapter.
Yet there are other ways by which we may cut ourselves off from God's forgiveness in the Holy Spirit.
One great sin against the Holy Spirit by which we might cut ourselves off from God's forgiveness is called Presumption: for example, when a person treats God's forgiveness as so automatic that they contemptuously go ahead with their evil plans.
Some people hear about presumption, blasphemy against the Holy Spirit, and unforgivable sins and they instantly worry that they may have committed or might commit one of these unforgivable sins.
Actually, the ability to worry about this, to feel remorse, and to ask for God's help is in and of itself a sign that the channel of grace is still open.
In fact, if one takes that worry too far, one risks the other classic unforgivable sin: Despair - specifically, despairing of God's forgiveness.
We should be careful not to blaspheme. We should be careful not to presume upon the grace of forgiveness nor should we ever despair of God's forgiveness.
We should remain focused on always trying to know and do the right thing, with humble penitence and faith in God.
(from a previous post)