Why would Jesus not introduce himself to the entire world, while he was here? An isolated "Messiah" is non-sensible.
I agree that Jesus was not an isolated messiah. He was active in his preaching and he traveled extensively to spread his message. Travel, writing, word of mouth-those were the tools at his disposal. In addition he charged his followers, and there were more than twelve, to go out an be "fishers of men." While other faiths are growing, Christianity is still going strong, and we're still charged with spreading the word.
According to the Book of Mormon, Jesus did appear to the Native Americans. Mostly, this shows little more than John Smith's contempt for the intellect of the native peoples.
However, in all practicality, a white or Middle Eastern man showing up in the Forbidden City in China would probably have been killed or sold something before he could begin to preach, miraculous demonstrations or no. Ditto India, ditto many parts of Africa, which at the time had fairly stringent policies of killing anyone perceived to be an interloper.
Some theologians argue that whoever the primary representation of God is in that country is really Jesus in disguise, but mostly they point to things that Christians have adopted from the other religions in the first place.
In an age of no mass media or high speed travel, I think Jesus did introduce Himself to the world using the tools He had at his disposal during His time. He traveled extensively, teaching about himself and the new covenant. He claimed to be the Messiah and was even executed for that crime. His executioner even put a sign in three languages on His cross that Jesus was the "King of the Jews", which is what "Messiah" means. Then, he entrusted His message to other people who would, in turn, teach other people by word of mouth. Even today, we know that word of mouth advertising is the most effective kind. We also tend to think of the disciples of Jesus as a small group of 12 men, but if you read the gospels carefully, there were hundreds, if not thousands, of disciples at the time of Jesus' death. All of them were put into service after his resurrection and Pentecost to spread the "Good News" of the gospels, that Jesus was the Messiah. If you look at the millions of people today and through the centuries who believed and still believe Jesus is the Messiah, I think you can say that those people did a pretty good job. In spite of great obstacles,the message is still being spread and Christianity is certainly no isolated faith today.