There's a few reasons, but number one for me is how incredibly clean the UI is.
Plex is a mess. Half of it is just premium shit they're trying to convince you to use. The actual "stream my own media" functionality is buried at the bottom of the menus.
Trying to get nontechnical family to use Plex was always a challenge, just because of how busy it is. I've never had this problem since moving to Jellyfin.