A former Respawn developer and a current one tried to break down the problem with Revenant's kit and his current pick rate. John Larson, Game Designer for Respawn Entertainment went in further detail on the philosophy of pick rate.
Since Revenant is known for a unique gameplay experience to other Legends, it can be hard to stay true to his nature while also trying to make it more effective for the meta. He needs to play like an assassin, but the game is team-based, so it's difficult to build his kit around team play while maintaining that fantasy.
Whether you agree with Larson's thought process or not, the results are the same. Revenant is in a terrible place. And based on his tweet, Respawn is working on something.
"So the best I can do is give you the PR answer of: we’re aware of the frustrations surrounding Rev, we know his pick rate is low af, and we’re motivated to do his thematic design justice with rewarding gameplay."
But based on the vagueness of the changes, Revenant may be the same for quite some time.