Because the user doesn't have a chat account yet.
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I just tried searching their username in the [list of all chat users](https://chat.stackexchange.com/users/?tab=all&sort=activity) and got nothing:

[![enter image description here][1]][1]

Only users who already have chat accounts can be superpinged just by using `@@<username>`. For a user who has a *site* account but not yet an account on Chat.SE, you need to superping them using the `<userID>@<site>` syntax.

  [1]: https://i.sstatic.net/bcl2z.png