Because he knows what Matthew is, and what Matthew’s source is. He doesn’t know what Mark’s source is, and Markan redactive tendencies aren’t nearly as clear as Matthew’s–he would have Matthew, and a point of comparison, making it obvious that Matthew was essentially making it up as he went.
He wouldn’t have the point of comparison for Mark–he’d have no reason not to take it prima facie.
The Gospels had not been written yet while Paul was alive. Paul was allegedly martyred sometime in the mid 60’s and the Gospel’s range from around 70-100 CE in their dates of composition (although some place Mark in the late 60’s).
Actually, Paul was not only unaware of the Gospels, his epistles say virtually nothing at all about Jesus as a person. He speaks only of the cosmic “Christ,” the crucifixion and the resurrection but he never mentions anything about Jesus’ ministry, his teachings, his alleged miracles or healings, the Virgin Birth or basically anything at all about what Jesus said or did while he was alive. He never even quotes Jesus except for a single ritualistic recitation at the Eucharist.
It is precisely this Pauline silence about any of the words or acts of Jesus of Nazareth which has led some people like Doherty to conclude that “Christ” started as a purely rhetorical, mythical invention of Paul’s and that the Gospels were fictions intended to provide Paul’s “Christ” with an earthly biography.
The theory isn’t as nuts as it looks at first blush, and while I don’t accept a conclusion that Jesus was purely mythical, I think that Doherty has shown that whatever roots the Jesus story had in a historical figure, that figure has been so overlayed with myth and revision and personal agendas that “Christ” has about as much to do with a historical Jesus as Santa Claus has in common with Nickolaus, the Bishop of Myra.
I probably should have added this above, when discussing Doherty’s argument from silence, but better late than never, the observation belongs to Peter Kirby.
2 Peter meets all of Doherty’s criteria for an epistle about the “heavenly Christ.” So much so, that Doherty himself uses it to show that the author was not familiar with any gospel narrative.
2 Pet.1.14 knows the gospel of John. Doherty proves too much.