So to answer your question: OpenSubsonic is only for music, and Jellyfin does much more than music. Adopting OpenSubsonic would mean maintaining two APIs, which doesn’t make a lot of sense.
Meanwhile Navidrome is adding Jellyfin API compatibility to get access to more clients :P
Wow, they’re not holding back!
And Bandcamp officially supporting OpenSubsonic clients wasn’t on my bingo card for sure!