Only Sith use "Darth" in their names. Also, in the time of the Republic they didn't always use their "Darth" names publically, to avoid detection.
After ROTJ, the Sith Order was dead, and many of their traditions lost with them.
Desann is a johnny-come-lately fallen Jedi, he's not a Sith. So there's no reason for him to take on the title, except ego (assuming he even knew anything about the Sith).