Why Does Obi-Wan Call Darth Vader “Darth” Only?

Obi-Wan Kenobi refers to Darth Vader as "Darth" in the original Star Wars trilogy because that was the name given to him when he turned to the dark side of the Force. "Darth" is a title used by Sith Lords, and Obi-Wan uses it to acknowledge Vader's affiliation with the Sith and the loss of his former identity as Anakin Skywalker. It also serves as a reminder of the darkness that has consumed him.