Text:
I consent to Plex to: (i) sell certain personal information (hashed emails, advertising identifiers) to third-parties for advertising and marketing purposes; and (ii) store and/or access certain personal information (advertising identifiers, IP address, content being watched) on my device(s) and share that information with Plex’s advertising partners. This data is used to deliver personalised ads and content, ad and content measurement, audience insights and product development. Your consent applies to all devices on which you have Plex installed. You can withdraw your consent at any time in Account Settings or using this page.
Soure: https://www.plex.tv/vendors/ (Might have to clear cache)
Can also read about the changes here: https://www.plex.tv/about/privacy-legal/
Probably because it works well, and has working clients on everything at this point. For some, a one-time fee was worth it when it was cheaper.
Sharing is also easier, as your friends just sign up to a plex account and you share your library with them. No need to send them an ip address and port, or fqdn that you have to maintain if your isp changes your ip address. It has its benefits, tbh, and the core sharing features still work for streaming. All the extra crap you can just turn off.
That why I think its still popular.