Well he was called that because His mother's name was "Mary" -- maybe that's what influenced the choice of "Marion". "Marion Robert Morrison" was John Wayne's original name. EXCEPT... His parents then had another baby boy, which they decided to name after Daddy this time. So, they then felt a need to change Marion Robert Morrison's name to "Marion Michael Morrison", so as not to overuse the name of "Robert".
So the second-born son gets named after Daddy, while the first-born son is stripped of Daddy's first name as his middle name and continues to wear a version of Mommy's name for his first name instead. If you catch my drift.