Why did they choose Barabbas?

The gospels tell us that under a custom honoured by the Roman authorities, a prisoner would be released by them at the time of the Jewish Passover. Pilate allowed the crowd to choose between Jesus and Barabbas. John Shelby Spong says in Jesus for the NonReligious, page 168, that in his research, he has been able to find no evidence that there was a custom of releasing a prisoner at the time of the Passover. He says the original crucifixion story could have been related to Yom Kippur because in this tradition, one lamb or goat was killed for our sins and one had the sins of the people symbolically transferred to him, after which he was chased away.

Historian Richard Carrier agrees with Bishop Spong that Barabbas took the role of the Yom Kippur scapegoat. His view, available online as part of a review he wrote for a book by Dennis R. McDonald, is that Rome would never have allowed such a practice:

[A] practice [of releasing a prisoner] could hardly have been approved by Rome, since any popular rebel leader who happened to be in custody during the festival would always escape justice.

Of course, Barabbas means "son of the father," and thus is an obvious pun on Christ himself. He also represents the violent revolutionary, as opposed to the very different kind of savior in Jesus (the real "Savior") . . . The Jews thus choose the wrong "son of the father" who represents the Old Covenant (symbolized by the rainbow, and represented by the ideal of the military messiah freeing Israel), as well as the scapegoat (the lamb) sent off, bearing the people's sins into the wilderness, while its twin is sacrificed (Lev. 16:8-10, 23:27-32, Heb. 8-9).

Although Bishop Spong's research suggests there was no such tradition, the gospel account tells us that the Jews were given the choice in releasing a prisoner for Passover, between Jesus, the Son of the Father, and Barabbas, the "Son of the Father". The pun is that they chose the wrong Son of the Father.

Jesus speak of things against the traditions and rituals practices of the Jews. Some may blamed this for reason why Jesus was rejected and claimed for Barnabas. But it would be more realisable to suppose that Jesus was fail with the expectation of the Jews who were suppressed by the Roman. In this regard Barnabas had to be chose because he was revolutionary, problematic to Roman not to the Jewish Priests. THe Jews expected that kinds of leader who can liberated them from Roman domination where Jesus mission was much contradicted. In this way, Jesus failed the hopes of the Jews which ultimately resulted in his rejection and open a way for sinner to release in place of him.

John 12:12-13 (KJV) ¹²On the next day much people that were come to the feast, when they heard that Jesus was coming to Jerusalem, ¹³Took branches of palm trees, and went forth to meet him, and cried, Hosanna: Blessed is the King of Israel that cometh in the name of the Lord.

When Jesus came into Jerusalem on Palm Sunday, the people welcomed Him as a deliverer from the oppression of Rome. They cried, "Hosanna!" which means "Save us!" They threw down palm branches, which since the time of the Maccabean revolt had been a Jewish symbol of victory (see section "3. Palm Branches" from Bible Encyclopedia and 1 Maccabees 13:51).

John 12:14-15 (KJV) ¹⁴And Jesus, when he had found a young ass, sat thereon; as it is written, ¹⁵Fear not, daughter of Sion: behold, thy King cometh, sitting on an ass's colt.

This prophecy is from Zechariah 9:9, and if you read the entire passage, you can see again what the Jews expected: a Messiah that would drive out their oppressors and be a physical savior.

Why did the Jews choose Barabbas instead of Jesus mere days after they welcomed Him so strongly? The other answers correctly point out that the chief priests and elders stirred the crowd against Jesus. However, the people were able to be swayed because their expectations were disappointed. Jesus didn't ride into Jerusalem on a battle charger, accepting a crown and forming an army to defeat Rome. Instead He came humbly, on a donkey, and He came not to conquer Rome, but to conquer death and sin through His sacrificial death.

They were persuaded by the chief priests and elders:

But the chief priests and the elders persuaded the crowd to ask for Barabbas and to have Jesus executed.

Matthew 27:20

They (chief priests and elders) had enormous power over the people since the synagogues and temple were the way people could stay in relations with other people (it was the heart of the community) and, they thought, with God. Perhaps the crowd felt threatened with losing their livelihoods and eternity with God.

But that is speculation - the text simply says that they persuaded the crowd.