They didn't come to christ, christ came to them, aggressively, the way he always does, with no respect for any other culture or way of life. It is a viral meme with an extremely ignorant, reckless and violent history.
Why must Christendom be spread to isolated tribes? Because nobody else is gullible enough to believe it. It's the last available market for those selling dumb. In the secular world's free market of ideas, Christianity is, rightfully, hemorrhaging members. They are relegated to seeking membership elsewhere, where people have never heard the name "Darwin" and don't understand they're being groomed for shame, pedophilia and theft. Oh, what a surprise Jesus holds in store for them, what a horrible surprise.
You wish.