Because the Bible says people who blaspheme the Holy Spirit won't be forgiven.
Oh, you are referring to Mark 3:29 as well as the similar verses found in Luke and Matthew.
I am reading from Mark 3:28-29. Please note that this is my interpretation but you will need to come to your own conclusion.
Mark3:28-29 says:
I tell you the truth, all the sins and blasphemies of men will be forgiven them. But whoever blasphemes against the Holy Spirit will never be forgiven; he is guilty of an eternal sin.
My Interpretation:
God does not contradict Himself - otherwise, he cannot be God. The Bible is God-breathed so it cannot contradict itself either. However, here are two verses one after another that seems to be self-contradictory. In verse 28, we are told that all our sins and blasphemies will be forgiven but in verse 29, there is an exception. If verse 28 is true, then verse 29 cannot be true and vice versa.
When you read it in context, then we learn that the teachers of the law are saying that Jesus is demon-possessed. In fact, in verse 30, it is said that [Jesus] said this because [the teachers of the law] were saying, "He has an evil spirit." What they were essentially doing was denying Jesus - they were not accepting Him. We know that to accept Jesus is part of the basis of obtaining our salvation. They did not. Instead, they were calling Jesus (who is also God) as being demon-possessed and that the miracles were the work of the devil. They were therefore, showing irreverence and contempt towards Jesus (that is the definition of blasphemy). God is triune. He is God the Father, God the Son (Jesus), and God the Holy Spirit. To deny Jesus is to blasphemy God and God as the Holy Spirit. The pharisees did not accept Jesus (they denied Him into their lives and even said that He was demon-possessed). That shows that they were not saved. Because they were not saved, they will naturally not receive eternal life and that in itself is the punishment. The act of not believing in Jesus is the unforgivable sin. You and I, we have accepted Jesus, asked for our sins to be forgiven, and asked Him to enter into our lives.
I do not believe that you have or will ever commit the "unforgivable sin" (unless you have not accepted Christ as your Saviour). But keep in mind that I believe in "once saved, always saved" and that all sins, past, present, and future are forgiven when we accepted Jesus as our Saviour. However, you have a responsibility to seek out the truth to be convinced in your own mind.
Jack Kelly from gracethrufaith.com says this in a better way than I do. You can read it here:
http://gracethrufaith.com/ask-a-bible-teacher/the-unforgivable-sin/ .