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Because Audino is both unique and at the same time extremly irrelevant.
Had they given a mega to pretty much any other Unova pokemon and people would raged about why that particular pokemon get a mega and others didn't.
But since Audino doesn't have a counterpart, or impressive stats, or an important role in any game, it getting a mega doesn't make you think "x should have gotten a mega too!", like for example Mawile and Sableye or Gyarados and Milotic.
