Why did Christianity spread so much more rapidly than other religions of salvation in the Roman Empire?
The Christianity had the royal patronage. It spread because it was spread with human effort and coercion; not because it had some great virtues as compared to other religions you have mentioned. Many people of the Roman Empire did not have scope to know much about the other religions. Buddhism also spread because of Missionary propaganda, not because people were attracted spontaneously for its inherent worth.
