Consider why the Roman public, commoners in particular but not exclusively, were so ready to abandon the religious beliefs of their forefathers and throw it all away, even defacing the old temples, to adopt some jewish desert hippie's promise of simple salvation. Perhaps you'd like to think this conversion of Rome was all by the sword, but in reality the early christian converts chose despite very credible threats of state violence, and the state itself only converted when the critical mass of common christians could no longer be denied or ignored.

Rome's culture and traditional was fundamentally broken; it no longer served the needs of the Roman people, and if Christianity hadn't popped up, it would have been some other system of reform instead. The status quo was unstable, rapidly deteriorating. You may idealize the religious tolerance of their polytheism, but what that matter if it isn't actually serving the spiritual needs of the people?