Jellyfin is written in C#.

I never understood why they don't support the subsonic protocol.

I mean... you might as well ask why the DLNA plugin has been broken for years. Or why they don't add a custom HTTP header to the client app so people can lock access in their reverse proxy.

As usual it's a mix of not enough developer manpower and "I don't use that so it's not a priority" i.e. the whimsies of FOSS.

Jelllyfin + Subfin (https://github.com/williamkray/subfin-plugin) works well, I've been using it with the OpenSubsonic Android apps listed in https://www.navidrome.org/apps/?platform=android&api=opensubsonic without issue. e.g. Substreamer, Tempus, Airdrome, etc. all work fine with Jellyfin + Subfin.

I'm guessing you're not on Android or iOS since Jellyfin has apps on those platforms - but you should be able to use any Subsonic/OpenSubsonic app once set up with the plugin.

What phone do you have that the jellyfin app and the jellyamp app are unavailable?

Edit: Also you can support subsonic with jellysub acting as a go-between. Considering its not a music-first solution, I'm not surprised that it doesn't support subsonic natively, personally.

Edit 2: Its unmaintained but works as of the last time I tried it out, fwiw. It isnt exactly a complex codebase either if something got tripped up by minor changes.

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