Peter’s failings in general have a particular emphasis in Matthew that we don’t find in Mark.
(Partly reproducing what I wrote in another thread here) That’s the key here, really. Overall, Matthew, due to his desire for certainty (Matthew has this dualistic worldview where the world is clearly defined into two opposing camps: those who follow Jesus and those who don’t, which stands in stark contrast to the ambiguity in Mark’s gospel), has a more positive portrayal of the disciples, unlike Mark’s blind buffoons. We can see this contrast in the way Mark and Matthew tell the same episode, that of Jesus explaining the parable of the sower.
Mark (4:10-13)
And when he was alone, those around him with the twelve asked him about the parables. And he said to them, “To you has been given the secret of the kingdom of God, but for those outside everything is in parables, so that
“they may indeed see but not perceive,
and may indeed hear but not understand,
lest they should turn and be forgiven.”
And he said to them, “Do you not understand this parable? How then will you understand all the parables?”
Matthew (13:10-17)
Then the disciples came and said to him, “Why do you speak to them in parables?” And he answered them, “To you it has been given to know the secrets of the kingdom of heaven, but to them it has not been given. For to the one who has, more will be given, and he will have an abundance, but from the one who has not, even what he has will be taken away. This is why I speak to them in parables, because seeing they do not see, and hearing they do not hear, nor do they understand. Indeed, in their case the prophecy of Isaiah is fulfilled that says:
“‘“You will indeed hear but never understand,
and you will indeed see but never perceive.”
For this people’s heart has grown dull,
and with their ears they can barely hear,
and their eyes they have closed,
lest they should see with their eyes
and hear with their ears
and understand with their heart
and turn, and I would heal them.’
But blessed are your eyes, for they see, and your ears, for they hear. For truly, I say to you, many prophets and righteous people longed to see what you see, and did not see it, and to hear what you hear, and did not hear it.
Whereas Mark simply writes that the disciples had been given “the mystery of the kingdom of God,” Matthew, true to form, clarifies it further: “To you it has been given
to know the mysteries of the kingdom of heaven,
but to them it has not been given.” Whilst Mark’s Jesus asks His disciples if they understood the parable (with an implied and understood “No” on their part), Matthew’s Jesus commends them: “blessed are your eyes, for they see, and your ears, for they hear.” Not only that - they also confidently reply to Jesus that
yes, they understood what He was saying (14:51-52):
“Have you understood all these things?” They said to him, “Yes.” And he said to them, “Therefore every scribe who has been trained for the kingdom of heaven is like a master of a house, who brings out of his treasure what is new and what is old.”
For Matthew, solid knowledge is a necessity, and he gives clear instructions on how to achieve it. Whereas Mark puts into question the easy categorization of ‘insiders’ and ‘outsiders,’ Matthew leaves no doubt as to who ‘gets it’. The Matthean Jesus requires no less devotion than the Markan Jesus, but He requires less puzzling. To go back to the example of the two houses, Jesus’ reputation as leader rests upon those who hear His words and those who act on them. It is as if Matthew could not risk ‘tarnishing’ and calling into question Jesus’ legacy by showing the disciples as ignorant hearers.