In Catholicism, why is heresy a mortal sin?

The reason is in the definition of heresy. As you stated

"Heresy is the obstinate post-baptismal denial of some truth which must be believed with divine and catholic faith, or it is likewise an obstinate doubt concerning the same"

If some truth must be believed with divine and catholic faith, then one who refuses to affirm that truth cannot be said to have divine and catholic faith. If that one does not have divine and catholic faith, then he is spiritually dead, since he is ultimately refusing to believe what God has revealed. Only mortal sin causes spiritual death.

This has to be an obstinate denial, because mortal sin is something you can only do "on purpose," so to speak. If you have not been corrected by anyone in your heresy (if you don't know it's heresy), you have a genuine invincible ignorance, so you don't mortally sin. Once someone gives you an explanation and presents the correct doctrine and shows what the Church teaches, if you go on denying the truth after this, you are sinning.

In Catholicism, why is heresy a mortal sin?

According to the Catholic Church the sin of heresy is a mortal sin because it is destructive to the virtue of Christian faith and it’s revolt against a Divinely constituted authority.

It can at times seem complicated.

Gravity of the sin of heresy

Heresy is a sin because of its nature it is destructive of the virtue of Christian faith. Its malice is to be measured therefore by the excellence of the good gift of which it deprives the soul. Now faith is the most precious possession of man, the root of his supernatural life, the pledge of his eternal salvation. Privation of faith is therefore the greatest evil, and deliberate rejection of faith is the greatest sin. St. Thomas (II-II, Q. x, a. 3) arrives at the same conclusion thus: "All sin is an aversion from God. A sin, therefore, is the greater the more it separates man from God. But infidelity does this more than any other sin, for the infidel (unbeliever) is without the true knowledge of God: his false knowledge does not bring him help, for what he opines is not God: manifestly, then, the sin of unbelief (infidelitas) is the greatest sin in the whole range of perversity." And he adds: "Although the Gentiles err in more things than the Jews, and although the Jews are farther removed from true faith than heretics, yet the unbelief of the Jews is a more grievous sin than that of the Gentiles, because they corrupt the Gospel itself after having adopted and professed the same. . . . It is a more serious sin not to perform what one has promised than not to perform what one has not promised." It cannot be pleaded in attenuation of the guilt of heresy that heretics do not deny the faith which to them appears necessary to salvation, but only such articles as they consider not to belong to the original deposit. In answer it suffices to remark that two of the most evident truths of the depositum fidei are the unity of the Church and the institution of a teaching authority to maintain that unity. That unity exists in the Catholic Church, and is preserved by the function of her teaching body: these are two facts which anyone can verify for himself. In the constitution of the Church there is no room for private judgment sorting essentials from non-essentials: any such selection disturbs the unity, and challenges the Divine authority, of the Church; it strikes at the very source of faith. The guilt of heresy is measured not so much by its subject-matter as by its formal principle, which is the same in all heresies: revolt against a Divinely constituted authority.

Catholic Culture explains it in it’s article on (Heresy, Schism and Apostasy) in a way that may more easily be understood.

The Catechism of the Catholic Church defines these three sins against the faith in this way:

2089 Incredulity is the neglect of revealed truth or the willful refusal to assent to it. "Heresy is the obstinate post-baptismal denial of some truth which must be believed with divine and catholic faith, or it is likewise an obstinate doubt concerning the same; apostasy is the total repudiation of the Christian faith; schism is the refusal of submission to the Roman Pontiff or of communion with the members of the Church subject to him." [Code of Canon Law c.751]

The Church's moral theology has always distinguished between objective or material sin and formal sin. The person who holds something contrary to the Catholic faith is materially a heretic. They possess the matter of heresy, theological error. Thus, prior to the Second Vatican Council it was quite common to speak of non-Catholic Christians as heretics, since many of their doctrines are objectively contrary to Catholic teaching. This theological distinction remains true, though in keeping with the pastoral charity of the Council, today we use the term heretic only to describe those who willingly embrace what they know to be contrary to revealed truth. Such persons are formally (in their conscience before God) guilty of heresy. Thus, the person who is objectively in heresy is not formally guilty of heresy if 1) their ignorance of the truth is due to their upbringing in a particular religious tradition (to which they may even be scrupulously faithful), and 2) they are not morally responsible for their ignorance of the truth. This is the principle of invincible ignorance, which Catholic theology has always recognized as excusing before God.

The same is true of apostasy. The person who leaves not just the Catholic Church but who abandons Christ Himself is materially an apostate. He is formally an apostate through willful, and therefore culpable, repudiation of the Christian faith.

Finally, the person who refuses submission to the Roman Pontiff, whom Vatican I defined as having a universal primacy of authority over the whole Church, is at least a material schismatic. It was thus common in the past to speak of the schismatic Orthodox Churches who broke with Rome in 1054. As with heresy, we no longer assume the moral culpability of those who belong to Churches in schism from Rome, and thus no long refer to them as schismatics.