First, let’s note that the Catechism (1864), following St. Augustine, says that the unforgivable sin is final impenitence, and we can rest in that.
As to why some theologians have considered envy of another’s spiritual good as a form of that sin, the answer turns out to be rather complicated. The short version is that it is connected with malice (as opposed to ignorance or weakness) and hardness of heart.
revert_jen already answered why it has been considered a sin against the Holy Ghost (cf. St. Thomas, S.T. IIb, Q. 36, a4). As to why some theologians have considered it unpardonable, see St. Thomas, S.T. IIb, q. 14. It’s too lengthy to quote here, and really needs to be read in its entirety, but here are some key excerpts:
God’s gifts whereby we are withdrawn from sin, are two: one is the acknowledgment of the truth, against which there is the “resistance of the known truth,” …] while the other is the assistance of inward grace, against which there is “envy of a brother’s spiritual good,” when, namely, a man is envious not only of his brother’s person, but also of the increase of Divine grace in the world.
In other words, it entails resisting divine grace. To elaborate further:
According to the various interpretations of the sin against the Holy Ghost, there are various ways in which it may be said that it cannot be forgiven. …]
…this may be understood to refer to the guilt: thus a disease is said to be incurable in respect of the nature of the disease, which removes whatever might be a means of cure, as when it takes away the power of nature, or causes loathing for food and medicine, although God is able to cure such a disease. So too, the sin against the Holy Ghost is said to be unpardonable, by reason of its nature, in so far as it removes those things which are a means towards the pardon of sins. This does not, however, close the way of forgiveness and healing to an all-powerful and merciful God, Who, sometimes, by a miracle, so to speak, restores spiritual health to such men.
Note that this is only one of several theories on what it means. The Church Fathers were not unanimous on this matter, and St. Thomas lists and expounds the major theories in the Summa passage linked above.