Currently, we can mention a user using '@' under comments, only if he/she has already commented in that thread. Why our Stack Exchange doesn't allow one to mention any other users in comments so that he/she gets attention of the question or discussion? Particularly, if somebody wants to get attention of a person who has more expertise in that topic?
It's a deliberate choice.
Many of the experts who provide some of the best answers don't want to be able to be paged.
They will visit the site when they can, and answer the questions they feel they should. But they have the complete right to ignore any question they like, for any reason.
When it's been proposed in the past to allow "pinging" a user who hasn't already expressed interest in a particular question, top users have threatened to leave if the feature were implemented.
Just guessing here, but one reason could be that user names are not unique. There are for example 12 "Joe", 10 "Dave", and that's just the first I checked.
Besides, it would seem a little weird because this place was never about individual users. That's one thing I really like about Stack Exchange.