In Christian historical movies, why aren’t the Romans speaking in Greek instead of Latin?

Because lots of Westerners know Latin (or at least know about Latin), relatively few Westerners know Ancient Greek, and Latin is the language Westerners associate with the Roman Empire. Having Greek spoken in a movie would really just confuse people, who’d expect the Romans in Palestine to be speaking Latin.

That, and the logistics of getting actors to speak Greek (and which Greek?), precisely because relatively few Westerners know Ancient Greek. Even the Passion of the Christ ended up going with Church Latin instead of Classical Latin…