Here's my theory on the bros' name ordeal. They don't HAVE last names. Ya know, like Cher --although she has a last name (several, I think), just no one knows it. Since they don't have a last name, the duo needs to be called something, so, Mario, being more well-known, was in their "title," hence the "Mario" Bros.
Like if, say Cher had a sister and the two teamed up as singers. They might be called the "Cher Sisters" 'cuz the former is (a lot) more well-known.
*Sigh* This is what I do. Pull theories out of my butt (figuretively) and try to defend them.
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If things don''t go your way, just keep complaining until your dreams come true.