Because they forgot bread

The episode where the disciples fail to bring bread along with them (Matthew 16:5-12) is organized in parallel:

A.Disciples forget bread (“take”)

B. Jesus: warnings of Pharisees and Sadducees

C. Disciples think it’s because they lack bread

A’. Jesus asks them about the two feedings (emphasis on what they “took up”)

B’. Jesus: leaven of Pharisees and Sadducees

C’. Disciples realize that it’s not bread but about teaching

Jesus’ speech in A’ is chiastic:

A. Littlefaiths, v 8a

B. Why do you discuss the lack of bread, v 8b

C. Feeding of five thousand, v 9

C’. Feeding of four thousand, v 10

B’. How is it that you don’t understand, v 11a

A’. Beware of leaven of Pharisees and Sadducees, v 11b

Several points here.  First, Jesus repeats the same warning twice (vv 6, 11) in virtually identical words.  The second time the disciples understand, and they understand because Jesus reminds them of events that they have experienced in their ministry with Him.  Lack of faith blinded their understanding; lack of faith kept them from discerning the metaphorical character of Jesus’ saying.  Second, the key to Jesus’ reminder is the repetition of the verb “take.”  The disciples forget to take bread, but Jesus reminds them of how much they “took up” (same verb) after the feedings of the multitudes.  There is always plenty of bread to take, always enough when Jesus is around.  If the disciples had faith, they would know that, and would not be distracted from Jesus’ words by concerns for things that Jesus abundantly takes care of.  Finally, the Passover/Exodus setting is important.  Jesus has just finished feeding 4000 people in a “desolate place” (15:32-39) and then He crosses the sea with His disciples on the dry land of a boat.  The disciples are worried about leaving behind the abundance of “Egypt,” but Jesus reminds them that He provides bread from heaven in the wilderness.