Those areas of the middle east had not been exclusively jewish (and not even then, really) since the Babylonian invasion and exile - 450 or so years prior.
In the intervening years, Israel was occupied by the Persians, Greeks, the Seleucids of Syria, a Hasmonean (jewish state) arose, and the Romans invaded. Some of these empires deliberately transplanted people groups into conquered territories to destroy ties to land and faith (thus, the higher levels of jewish society were exiled to Babylon, and many were made to work by Nebuchadnezzar as Babylonian officials - like Daniel, Shadrach, Meshach, and Abednego). When the exiles returned to Jerusalem under Cyrus to rebuild the temple (Ezra and Nehemiah), they faced opposition from groups (Samaritans, Ammonites, Arabs and Philistines) moved to Israel by the Babylonians.
By ~30AD, there were peoples of many faiths and cultures living in Israel. For devout Jews, being forced to tend swine would have been a humiliation (as noted in the story of the Prodigal Son). The Romans would certainly like pork, and would have no qualms about forcing them on their jewish vassals.