My theory, God waited until the Roman Empire was at its peak, because the infrastructure to facilitate travel around Europe, North Africa, etc. was there, philosophy had reached a point in the west that it could be synchronized with Christianity. If He had come earlier, and somewhere else, humanity wouldn’t have been ready to receive the message or have the means to spread it.
Also, the Asian cultures were too far removed, too pagan to receive God, same for American Indians.
The Romans, though pagan, had a virtuous form of paganism and philosophy that put them closer to God, than say the Carthaginians with their human sacrifice.