Given a set of conditions with an outcome favorable to L, even with a small advantage, it would eventually rule out R. So, I think it's very possible that ancient conditions were such that both L and R existed, but that one gained an advantage, not by pure chance, but because conditions favored one over the other. Conditions elsewhere in the universe may well tip the advantage to the mirror compounds.
The fact that conditions favored one over the other..
Was that not determined by chance?