Commentary for Avodah Zarah 16:16

Nahman b. Yitzchak now offers a new suggestion as to what changed with the exile of the Sanhedrin to Hanut. From this point forth, they no longer adjudicated capital crimes. But this change was not because of Roman persecution. It was because of Jewish corruption. There were so many Jewish murderers at that point that the courts could no longer properly try them. To avoid having to exile so many murderers, the court chose to go into a sort of “mini-exile.” For once their place of judgment had changed, they could no longer adjudicate capital crimes.