This is one of the more bothersome things about MacArthur. I don't understand why he went through such efforts to block Wainwright's CMH. Ironically, if you read MacArthur's own CMH citation it looks like the only thing wrong with it is the name it is attributed to. It should read Wainwright instead of MacArthur. I actually wrote an entire paper on just that odd bit for one of my masters degree classes.
This is a topic I haven't given a great deal of attention to but I suspect MacArthur being denied the MOH in the Veracruz expedition and again in WWI may have had some role to play.