Because Plex worked amazingly ~5-6 years ago, while Jellyfin was still very early stage. It'd sort of work. So there's a lot of "Well I'm already using Plex" inertia. Plus Plex has very lovely, polished clients, you can easily invite people, share your library with others etc etc. Once they got all that of course, then the changes/fees/ads/bloat started.

Jellyfin is great, it's all I've ever used. But it's still very rough - AppleTV support is limited+buggy (ask my wife...), you can't cast to a Chromecast from iOS etc. Plugins are required for some things to work. Transcoding is harder to get working. Good luck downloading stuff for offline use on the mobile client apps, all you get is the raw movie/tv show file - you have to use VLC to play it.

Again, I love Jellyfin and I use it daily. But we're both kidding ourselves if we think the polished experience you get with Plex is the same as it is with Jellyfin.