Again, great question. Thanks for a stimulating discussion.
My answer to why Yahwism didn't pop up anywhere else is that God intended Yahwism to center around Abraham and his progeny, ultimately revealing Himself in the person of Jesus—and this is mandatory to the faith. If Yahwism had popped up in the Americas, it would have been totally disconnected from the line of Abraham, Moses, David, and Christ, in which case, it wouldn't have been Yahwism. Yahwism needs not only the context of the covenant and the Sinaitic law, but also the prophetic line and writings, not to mention the genealogy, that lead us to Jesus. How could that possibly have developed like a quasi-parallel track in the Americas or Australia? Even at its best, it would be unconnected to the prophetic writings, the descendants of Abraham, the monarchy, and the Messiah. So, practically speaking and theologically speaking, how could that even work?
Instead, I ruminate on the musings of C.S. Lewis, who wrote (
Mere Christianity, p. 65), referring to John 14.6:
"Here is another thing that used to puzzle me. Is it not frightfully unfair that this ("No one can come to the Father except through Me") should be confined to people who have heard of Christ and have been able to believe in him? But the truth is God has not told us what his arrangements about the other people are. We do know that no man can be saved except through Christ; we do not know that only those who know Him can be saved through Him. But in the meantime, if you are worried about the people outside, the most unreasonable thing you can do is to remain outside yourself. ... If you want to help those outside you must add your own little cell to the body of Christ who alone can help them. Cutting off a man's fingers would be an odd way of getting him to do more work. ...
The key phrase for our discussion there is, "God has not told us what his arrangements about the other people are."
Bottom line is I trust that (1) God wants all people to come to Himself; (2) God has made plans to reveal Himself in some way to all people; (3) God is continually working to draw anyone who will come to Himself; (4) My lack of understanding or the incompleteness of my knowledge or theology doesn't negate God's work in the world or render it as inadequate; and (5) It has to be possible, somehow, that people who lived before Christ or who never heard of Christ have a way to be found by God.