Because, as Peter tells us, Paul's writings are difficult to understand. They may be (and often are) used to concoct nonChristian doctrines and antiChristian beliefs.
There is nothing wrong with Paul. There is everything wrong with Paulicians.
I would say that to be true of all the Scriptures but Paul, having written more, provides more opportunity.
Anyway if you see Paulicianism breaking out, give me a shout, I'd be interested.
From New Advent:
The cardinal point of the Paulician heresy is a distinction between the God who made and governs the material world and the God of heaven who created souls, who alone should be adored. They thought all matter bad. It seems therefore obvious to count them as one of the many neo-Manichaean sects, in spite of their own denial and that of modern writers (Ter-Mkrttschian, Conybeare, Adeney, loc. cit.; Harnack, "Lehrbuch der Dogmengeschicte", Tübingen, 1909, II, 528). But there is a strong Marcionite element too. They rejected the Old Testament; there was no Incarnation, Christ was an angel sent into the world by God, his real mother was the heavenly Jerusalem. His work consisted only in his teaching; to believe in him saves men from judgment. The true baptism and Eucharist consist in hearing his word, as in John 4:10. But many Paulicians, nevertheless, let their children be baptized by the Catholic clergy. They honoured not the Cross, but only the book of the Gospel. They were Iconoclasts, rejecting all pictures. Their Bible was a fragmentary New Testament. They rejected St. Peter's epistles because he had denied Christ. They referred always to the "Gospel and Apostle", apparently only St. Luke and St. Paul; though they quoted other Gospels in controversy.
And, from http://www.medievalchurch.org.uk/h_paul.php
Doctrines.- Little is known of the tenets of the Paulicians, as we are confined for information to the reports of opponents and a few fragments of Sergius' letters which they have preserved. Their system was dualistic. There are two principles, two kingdoms. The Evil Spirit is the author of, and lord of, the present, visible world; the Good Spirit, of the future worid. Of their views about the creation of man, little is known but what is contained in the ambiguous words of Sergius... This passage seems to teach that Adam's sin of disobedience was a blessing in disguise, and that a greater sin than his is the sin against the church... The Paulicians accepted the four Gospels, fourteen Epistles of Paul, the three Epistles of John, James, Jude, and an Epistle to the Laodiceans, which they professed to have. The Old Testament they rejected. They rejected the title of ... (mother of God), and refused all worship to Mary. Christ came down from heaven to emancipate men from the body and from the world, which are evil. The reverence for the cross, they looked upon as heathenish. The outward administration of the sacraments of the Lord's Supper and baptism, they rejected. Christ himself is our baptism. Their places of worship they called "places of prayer"... Although they were ascetics, they made no distinction in foods, and practised marriage.
I doubt any of us whom you casually call "Paulicians" hold to the same doctrines of this seventh century minority. Rather, it appears it is only a "spitwad" that gets launched whenever the epistles of Paul are used to straighten up misrepresentation of the Gospel.