Christ in the SynagoguePainting by Nikolay Ge, 1868
Nobody upvoted this response, which happens on Quora sometimes. In fact, almost nobody paid attention to the question at all. But I didn't think it was too bad for a short answer.
Q: Why did Jesus have to leave Nazareth to get his first followers?
A: Nazareth in Christ’s time was a tiny, insignificant village of around 400 people — not much of an audience. Also, as his quoting of a popular proverb (“not without honor, except in his hometown”) indicates, they were not inclined to see this carpenter, whom they’d watched grow up, as a possible Messiah. In fact, Luke's Gospel reports they were downright hostile! Add to that the rumor that he was illegitimate — born out of wedlock.
To expose his message to as large an audience as possible — and generate followers — it would have been necessary to base his operation in a larger, slightly more cosmopolitan town like Capernaum (population 1500+) and canvas all of Galilee, as the synoptic gospels (Matthew, Mark, and Luke) describe.
In addition, John’s gospel indicates that Jesus met his earliest and most important students (Peter, Andrew, and possibly John) in the crowds that gathered around John the Baptist, south of Galilee on the Jordan river.
To sum up, Nazareth was a small and rather hostile audience, but Capernaum, the Galilee region, and particularly John the Baptist’s hangers-on provided more fertile soil for Jesus’ unique message.