Tech issue. Just like other casting technologies, DLNA uses it’s own set of commands. Unfortunately, DLNA is not backed by any single company (Chromecast by Google, Airplay by Apple, etc.) so there aren’t any standards or updates. It’s an old tech that hasn’t changed in a long time. There are no plans to include DLNA at this time.
There is a feature request so go add your vote. PLEX as DLNA Control Point
