Because it fucks up subtitles on their Nvidia Shield and firestick apps, and has issues with transcoding too. If you're purely using the desktop app, or browser then it's better. But Plex just works in every aspect. Subtitles, transcoding, sharing libraries. Have had no issues or weird bugs. Anytime I try JF I have to dive deep into resolving bugs, some of which are unsolvable by me.