The current theory is that the moon got completely walloped on the near side by one or more very large impacts. These impacts later became the maria, and consequently that side of the moon was less massive. Or rather, its geometric center and its center of mass did not coincide.
Now any moon will become tidally locked over time. But when there's such a difference in density between one side and the other, it will lock with the "heavy" side facing away.
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