I always liked that dreadlords were basically THE vampires of Warcraft, insidious evil creatures, not the kind of cliche posh aristocratic vampires, then Blizz made their CREATOR and true lord exactly that. IMO Denathrius just doesn't hold the credibility as their master.
"At the BlizzConline 2021 Q&A, we learned that Sire Denathrius was originally meant to have a small role at the start of Shadowlands, but the team loved the voice actor's performance and decided to keep Denathrius alive for future stories. We last saw Sire Denathrius' essence trapped in Remornia following Castle Nathria."
He wasn't even supposed to be a big player but they put voice acting over their writing.