Judaism's monotheism applied to Jews. The god(s) of other peoples were all well and good, but never to be worshipped by a Jew, whose one god would never tolerate company. So the Romans could have as many gods as they wished. As for the Romans, the Jews -- poor chaps -- had to make do with just one very jealous god.

Christian (and Mohammedan) monotheism insisted on their god or the highway to hell. That made them against gods -- apolytheists, so to speak -- the worst form of atheism!