Like the fact that he didn't know of it.
The possibility that he did know about it seems no less valid. Eugene Rosenstock-Huessy suggests that Peter, supervising Mark, says to him, "Cut this out. This will do. Enough has been said".
The same applies to the Sermon on the Mount. Do you honestly think Mark would have left these things out if he had known of them? Furthermore, there are places where Mark seems to have extras compared with the other gospels
And this could rather indicate that Mark was using Matthew plus another eyewitness who could add more detail.
such as the story of the exorcism at Gerasa (the one where the demons move to a herd of pigs). However his extra bits tend to are more mundane. So you have to ask if Mark had a thing against the Beatitudes but liked pigs. It's much more likely that the revisions went the other way.
Well I think the reason for that is that Matthew wrote soon after the church was born. Compare with the birth of the nation of Israel. Not long after the Exodus they were given instructions on how to live as God's people, by Moses. Matthew was doing a similar thing. Mark is written as a fast-moving drama, so it cannot include long sermons.
I'm afraid I'm utterly convinced Matthew wrote first.
I asked Hope this question once but he failed to answer it: what are the names of the twelve apostles?
Simon, Andrew, James, John, Philip, Bartholomew, Matthew, Thomas, James son of Alphaeus, Thaddaeus, Simon the Zealot, Judas Iscariot.
Even if you can give me an unambiguous list using each of the four gospels, you still don't know if any of them were the sources of any of the gospels.
That's approaching the question from the pov of someone outside the church.
If you wanted to know how a horse behaves when someone climbs onto its back, who would you ask: a person who has studied horse anatomy and physiology and can name all their bones, or a person who has looked after horses since they were young? It's like that with the gospels. If you don't ask someone who is connected to them through the church, you won't get the truth.
At least we are back on topic though