It seems possible that the blame for this was put on the Jewish crowd that supposedly yelled "Crucify him!" that would have been a subjective way of blaming most Jews who refused to convert into "the Way". But who knows?
And at Passover this 'Jewish crowd' were pilgrims from all over the Mediterranean, to celebrate the festival of freedom from foreign domination, but upon arriving they would see many signs of Roman supremacy most of whom never heard of this Jesus when they 'yelled crucify him'.
According to Mark Jesus was quietly arrested at night on the Mount of Olives. This means that his foes were concerned about his general popularity and feared taking him into custody publicly (cf. Mark 14:2).
All we have for possible answers is an historical reconstruction. And included in that is always the interests of the gospel authors;
To have the Church accepted as a legitimate religion in the Roman Empire.
To argue for the Church's way of being Jewish in the aftermath of the Temple's destruction by the Roman in the year 70.
To explain why the Temple was destroyed. To show that the Church's claim that the Crucified One has been raised is consistent with the Scriptures of ancient Israel.
In the case of the passion narratives, these factors contributed to a tendency to de-emphasize Roman responsibility and to highlight the role of Jewish figures in bringing about the execution of Jesus.