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.

Why did they choose Barabbas?
Cityscape Web Enhancement lists an alternate class feature for Druids and Rangers, called Urban Companion:


  The character gains the companionship of a smaller but far more intelligent creature than she otherwise would have. This is identical to the sorcerer's ability to summon a familiar (PH 52), including all benefits granted and gained by the familiar, except as noted below. Her functional level for determining the abilities of the companion is equal to her druid level or one-half her ranger level.


Does a character with Urban Companion qualify for the Improved Familiar feat?
