The problem is that Dark Pit doesn't really have a different playstyle from Pit, unlike Lucina, Dr. Mario, Young Link, or even Pichu. All but four attacks (five if you include his final smash) are identical to Pit's, and many of those four attacks are outright worse than what Pit has or have differences that are so marginal that they are difficult to notice.
If he returned, that could be an opportunity to change up his moves and make him more worthwhile as a separate character, but as he currently is, he doesn't really add much.