Because that’s the way it is defined. Look in any modern dictionary. It is always accompanied by the qualifier “offensive” or “derogatory.”
The term derives its negative connotation from how it was used during the Protestant Reformation. Here are some examples:
Yet it is the duty of Christians to marry only in the Lord. And, therefore, such as profess the true reformed religion should not marry with infidels, Papists, or other idolaters: neither should such as are godly be unequally yoked, by marrying with such as are notoriously wicked in their life, or maintain damnable heresies. Westminster Confession of Faith, CHAPTER XXIV.
If, perchance, there were reasonable Papists we might speak moderately and in a friendly way, thus: first, why they so rigidly uphold the Mass. For it is but a pure invention of men, and has not been commanded by God; and every invention of man we may [safely] discard, as Christ declares, Matt. 15:9: In vain do they worship Me, teaching for doctrines the commandments of men. Smalcald Articles, Part II, Article II.
Why would you wish a Catholic to identify with a term that is defined as offensive and derogatory?