Probably for similar reasons as to why they moved from Reddit. Also configuring their own instance to approximate a traditional forum would honestly kind of undermine the whole point of using Lemmy or the like to begin with (at least imo).

I understand the sentiment of wanting them to to make their stuff easier to follow & post to from here and other places in the Fediverse, but from what they wrote, I get the sense that this format simply isn’t what they were ever looking for in terms of fielding discussions/questions. Their move to Reddit was more of a compromise for where they were at with the project at the time, but now that Jellyfin’s more developed in terms of the software and community, a forum is a more workable prospect.

Lemmy’s moderation tools are severely lacking and they seemed to want to get away from the rank by voting system and the churn created by older but relevant and active discussion being hidden on Reddit and Lemmy.

Some forums have nested comments. It depends on the software.