Whatever the reason it is not because of startup wear since that's relatively unimportant. All oils provide sufficient MOFT at startup since all are very thick. Many 0W oils are thicker at most temperatures than their 5W counterpart, only at temperatures below -35F or so are they guaranteed to be thinner. But at these temperatures both are very thick.Then the question is why some cars call for 5W20, not 0W20? Or vise versa?
Some people have speculated that one reason for listing a 0W rated oil in the owner's manual is that these oils are nearly all synthetic and therefore can meet the manufacturer's OCI targets. Who knows if that is correct.