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.