Because the mob that demanded His crucifixion was composed of Jews. The Jewish authorities at the time were not permitted to impose the death penalty, so they had to goad the Roman authorities. Unruly mobs are very effective at persuading weak government officials. Think we Americans are immune? I got two words for you: George Floyd. In any case, Pontius Pilate is one of two human persons mentioned in the Nicene Creed. In it, he is assigned blame for Jesus' crucifixion.
So ... reality is maybe a little more complicated than your question implies.
But why would one expect a True Believer to have read the New Testament? Sheesh!
The same mob that welcomed Him into Jerusalem as a hero five days earlier.
His own Jewish disciples thought of him as a Rabbi, not the Messiah, but what would a lapsed Roman Catholic understand about Christianity anyway?
24 So when Pilate saw that he was gaining nothing, but rather that a riot was beginning, he took water and washed his hands before the crowd, saying, “I am innocent of this righteous man’s blood; see to it yourselves.” 25 And all the people answered, “His blood be on us and on our children!” 26 Then he released for them Barabbas, and having scourged Jesus, delivered him to be crucified.