Because Game Freak forgot to unban these items.
The Pokemon Company are offering the Mewtwonite X and Mewtwonite Y Mega Stones in Pokemon Sun and Pokemon Moon via a special code. Those items will allow Mewtwo to Mega Evolve into Mega Mewtwo X or Mega Mewtwo Y respectively. Here's the directions to acquire those items:
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Select Mystery Gift on the main menu in either Pokemon Sun or Pokemon Moon.
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Select Receive Gift.
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Select Get with Code/Password, then Yes, and then Yes again to connect to the Internet.
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Enter your code.
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Watch as you receive your Mega Stones.
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Speak to the deliveryman in any Pokemon Center to pick up your Mega Stones.
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Save your game!
There's just one problem: Game Freak forgot to unban those items in online matches that allows special Pokemon! Whenever I tried to play with Mega Mewtwo X or Mega Mewtwo Y I received the following message:
"There is at least one Pokemon with a banned held item."
March 6, 2017 update: The regulations have been updated in Pokemon Sun and Pokemon Moon to accept the Mewtwonite X and Mewtwonite Y Mega Stones in Special Free Battle matches.