Why had Barabbas been put in prison?

Answer: Murder and insurrection

Barabbas had been imprisoned for an insurrection in the city and for murder, along with the rebels of that uprising.

“(Barabbas had been imprisoned for an insurrection in the city, and for murder.) And a man named Barabbas was imprisoned with the rebels who had committed murder during the insurrection.”— Luke 23:19; Mark 15:7 (BSB)