Because Irenaeus called him the Father of all heresies, the Father of a counterfeit Christianity that emerged there in Rome during the reign of Claudius and Nero. And by the end of the first century 50 different religious cults had emerged from him. All of these cults were infiltrating the Church with their magical heresies, rituals, icons, and pagan practices.
The church fathers from Justin Martyr, to Irenaeus to Tertullian, to Origen, to Hippolytus to Eusebius spent more paper on Simon Magus and his sojourn in Rome than on anyone else. And we have this from Eusebius about the Simonians in his day:
11.". . . he [Simon Magus] feigned and counterfeited faith in Christ, even going so far as to receive baptism. 12. And what is surprising, the same thing is done even to this day by those who follow his [Simon Magus's] most impure heresy. For they, after the manner of their forefather, slipping into the Church, like a pestilential and leprous disease greatly afflict those into whom they are able to infuse the deadly and terrible poison concealed in themselves. The most of these have been expelled as soon as they have been caught in their wickedness, as Simon himself, when detected by Peter, received the merited punishment. . . .
6."We have understood that Simon was the author of all heresy. From his time down to the present those who have followed his heresy have feigned the sober philosophy of the Christians, which is celebrated among all on account of its purity of life. But they nevertheless have embraced again the superstitions of idols, which they seemed to have renounced; and they fall down before pictures and images of Simon himself and of the above-mentioned Helena who was with him; and they venture to worship them with incense and sacrifices and libations." [Eccl Hist, Book II, Ch 1, Sect 11,12... Ch 13, Sect 6]
Those pagan practices from the disciples of Simon Magus eventually found a home in the Church of Rome just as his bones found their resting place in that pagan graveyard beneath the Basilica of St Peter.
OK, so he is a heretic. Let's dismiss him. I would rather talk about true faith, wouldn't you?