Let’s start with the silly question approach.
Q: Why did Luke not write that Jesus was comforted by an angel after his testing in the wilderness after his baptism?
A: He needed an angel to comfort Jesus in Gethsemane and he only had one angel to work with.
So…imagine that each of the gospel writers has a card that represents each element of the tradition. Everybody has a cleansing of the temple card, a healing the blind man card, an unfruitful fig tree card and so on. And each has some cards that the others don’t have.
You play your “cards”—cite the various elements of the tradition you know—at the place in the narrative where it will do what you want. Matthew makes a collection of the teachings of Jesus and has him deliver it on a mountain to make the parallel with Moses unmistakably clear. Luke puts his very similar collection on a plain, surrounded by a crowd of people. Notice the insubstantiality of the wilderness angel. Keep it in mind when we get to a representation of the Gethsemane angel.
Each writer will play these cards at the place in the narrative where they say something important about Jesus. There is a good deal of variability in where these cards are to be played, but you have only so many cards and once you have played a card, it is gone. So you have to use each card thoughtfully.
The “play the angel card” metaphor has some important contributions for us. For people who have been reading the gospels as if they were newspaper accounts, the notion that there is a strategy of presentation in each of the accounts can be really good news. It turns the differences in the accounts from “discrepancies” into “variations” No one imagines on hearing a “theme and variations” piece , that one of the variations is “correct” and that the others are not. So the “angel card” approach begins by saying, “Remember, this is not journalism.”
Or, to say the same thing another way, the accounts we read in the gospels are not journalistic accounts. A journalist who was covering Jesus’ ministry would (could) just say where Jesus went and what he said. He could interview the crowds or the Pharisees or the beneficiaries of the miracles and get them to say how they felt about it. 
But in addition to removing the misunderstanding that these are newspaper accounts, the “angel card” metaphor also prompts a more thoughtful attention to the narratives each writer develops. Matthew, for instance, maintains the tie to Judaism more closely than the others. Luke is very severe about wealth and poverty. John understands Jesus as the pre-existent Logos and fully in charge of the events of his life, including his crucifixion. Watching what cards are played and where in the narrative each is used provides a much richer sense of the ministry of Jesus.
Luke’s Angel Card
So we see that Luke did not provide an angel to comfort and restore Jesus after his wilderness tempting by the Devil. And now we can speculate that he was “saving his angel card.” for Gethsemane, when Jesus would be in great need of it. No angel appears in Mark and Matthew. Jesus is prostrate on the ground and virtually paralyzed by the prospect facing him.  In Luke, Jesus in kneeling (not prostrate) and is ministered to by an angel who functions as a trainer might in preparing an athlete. 
Luke’s angel changes everything. Jesus’ healing ministry continues through the arrest and the interrogations and the trials and even during the crucifixion. In Jesus’ forgiveness of the penitent criminal on the cross, I see the effects of the strengthening angel. Jesus’ last words in Luke are not “My God, why have you abandoned me?”, but “Father, into your hands I entrust my spirit.” In that, I see the effects of the strengthening angel and it makes me grateful that Luke saved his angel card until then.
I started with the silly question, but clearly, the “play your card” metaphor is a different approach entirely. The gospels are compositions. Their shape is determined by what cards each narrator has available and where he chooses to play them. That perspective invites us to a much richer appreciation of the narrative traditions of the ministry of Jesus. Every variation gives another chance to appreciate the underlying theme.
 Of which, I must say Brahms’ Variations on a theme by Haydn (also called the Saint Anthony Variations), is my favorite.And there is fugue at the end, just for fun.
 If Herod, whom Jesus once called “that fox,” (Luke 13:32) had a network of spies covering Galilee for him, it would be a Fox Network. Very likely the first one.
 And it doesn’t get better. In Mark and Matthew, Jesus’ last words from the cross are, “My God, why have you abandoned me?”
Raymond E. Brown argues, in The Death of the Messiah that the “agony” of Jesus in the garden represents the Greek agonia, which is a state of extreme readiness in preparation for an athletic event.