Why did Jesus cause the fig tree to wither?

The events of the fig tree in Mark 11 are part of a larger narrative and must be understood in context. The context of Mark 11 is Jesus entering Jerusalem shortly before His crucifixion during the week of Passover. The nation of Israel had been offered the opportunity to receive their Messiah and enter into the promised Kingdom, but they rejected Jesus and now they were about to kill their Messiah.

In place of faith, the nation was displaying hypocrisy and unbelief, so in his chapter Mark records Jesus entering Jerusalem and the temple only to find sin and unbelief rather than prayer and faith. The temple gave the appearance of holiness within Israel, but it was merely an illusion and hypocrisy.

So, as Jesus entered into the city, He chose to use His encounter with the fig tree as an opportunity to teach the disciples a lesson concerning the future judgment coming for this unbelieving generation of Israel by the power of God.

Mark 11:12 On the next day, when they had left Bethany, He became hungry.
Mark 11:13 Seeing at a distance a fig tree in leaf, He went to see if perhaps He would find anything on it; and when He came to it, He found nothing but leaves, for it was not the season for figs.
Mark 11:14 He said to it, “May no one ever eat fruit from you again!” And His disciples were listening.
 

As Jesus approaches the fig tree the first time, Mark says Jesus was hungry and looking for fruit on the tree. The tree had leaves, indicating that it was ready to produce fruit, but Jesus finds none. Importantly, Mark notes that this was not the season for figs, so it was not surprising that the tree was without fruit. Jesus then declares that this tree will never again produce fruit.

At first glance, we might assume that Jesus' response to the tree for failing to produce fruit out of season was unfair and even spiteful, however Mark informed his readers that this was not the season for figs to preclude that conclusion. Jesus knew the tree wouldn't have fruit, so He must have orchestrated this moment to teach His disciples a lesson.

Jesus statements to the tree were a demonstration conducted for the sake of the disciples to teach a spiritual truth concerning the generation of Israel who had rejected His claims to be Messiah. Israel is often pictured in Scripture as a fig tree, and this tree becomes a picture of the generation of Israel in Jesus' day who rejected His clam to be Messiah. Though this generation in Israel gave an outward appearance of piety and religious belief (just as the tree appeared to be in season for fruit), in reality Israel was producing no spiritual fruit by their unwillingness to believe in the Messiah they claimed to seek.

Therefore, Jesus condemned the fruitless tree as an picture of what would happen to this unbelieving generation of Israel. If the Lord was willing to condemn a fruit tree for failing to produce fruit out of season, how much more will He bring judgment against Israel for failing to show spiritual fruit in the face of clear evidence that Jesus was the Messiah? Jesus Himself says He will judge the generation of Israel that rejected Him by condemning it greatly:

Luke 11:49 “For this reason also the wisdom of God said, ‘ I will send to them prophets and apostles, and some of them they will kill and some they will persecute,
Luke 11:50 so that the blood of all the prophets, shed since the foundation of the world, may be charged against this generation,
Luke 11:51 from the blood of Abel to the blood of Zechariah, who was killed between the altar and the house of God; yes, I tell you, it shall be charged against this generation.’
 

Before returning to the tree, Mark records Jesus' experiences in the temple. Jesus chastised the money changers and reminded Israel that God's House was to be a house of prayer not a den of theives. Mark's account reinforces for the reader how evil was this generation and how hypocritical was Israel.

Mark 11:15 Then they came to Jerusalem. And He entered the temple and began to drive out those who were buying and selling in the temple, and overturned the tables of the money changers and the seats of those who were selling doves;
Mark 11:16 and He would not permit anyone to carry merchandise through the temple.
Mark 11:17 And He began to teach and say to them, “Is it not written, ‘ MY HOUSE SHALL BE CALLED A HOUSE OF PRAYER FOR ALL THE NATIONS’? But you have made it a ROBBERS’ DEN.”
Mark 11:18 The chief priests and the scribes heard this, and began seeking how to destroy Him; for they were afraid of Him, for the whole crowd was astonished at His teaching.
 

The judgment Jesus promised for this generation is pictured by His second encounter with the fig tree:

Mark 11:19 When evening came, they would go out of the city.
Mark 11:20 As they were passing by in the morning, they saw the fig tree withered from the roots up.
Mark 11:21 Being reminded, Peter said to Him, “ Rabbi, look, the fig tree which You cursed has withered.”
 

When Jesus visits the fig tree a second time, the tree is withering starting from the roots. Jesus' condemnation of the tree initiated a slow dying process starting at the roots, the tree's source of nourishment. As the tree lost nourishment from the roots in the ground, it began to die in keeping with Jesus' words.

The withering tree completes Jesus' picture of Israel. Israel would suffer judgment in the same way the tree withered: from the roots up, so to speak. Jesus' condemnation would result in a gradual withering of this generation in Israel by the cutting off of all spiritual nourishment from God. In the Luke 11 passage quoted above, Jesus said this generation of Israel would reject the word of God delivered by the prophets and apostles sent to her. Since spiritual nourishment is found in the word of God (Matthew 4:4), this generation would be spiritually starved by their rejection of the Gospel. Eventually, this generation would suffer physical judgment when the city of Jerusalem was destroyed in AD 70.

Mark 11:22 And Jesus answered saying to them, “ Have faith in God.
Mark 11:23 “ Truly I say to you, whoever says to this mountain, ‘Be taken up and cast into the sea,’ and does not doubt in his heart, but believes that what he says is going to happen, it will be granted him.
Mark 11:24 “Therefore I say to you, all things for which you pray and ask, believe that you have received them, and they will be granted you.
 

Upon seeing the withered tree, the disciples exclaimed that that the tree was dying. Their exclamation carried a tone of disbelief, which led Jesus to launch into a teaching on the power of prayer. When the disciples pray in God's will, Jesus taught they are praying with the power of God. When Jesus spoke his words of condemnation to the tree, His words had the power to wither the tree because they were consistent with God's will. Jesus incarnate as man understood the Father's will through the Spirit, just as all men can, only Jesus' understanding was perfect, so His prayers had great power to effect change.

Jesus taught the disciples that their ability to see great things happen from their prayers was a matter of being confident of God's will beforehand, and when we pray in God's will, we will see God's power at work in the result.