Well idk about a 100% free phone os like trisquel but there is GrapheneOS but that one only works with Pixel phones. And some banking apps don't work so you have to check if your banking app works if you use a banking app, and google pay + NFC i'm pretty sure does not work. But everything else should work out of the box. There's also lineageOS that works on more phones then just Pixel phones so that might be an alternative.
I’d say one big reason is that Nostr isn’t just one app, it’s a protocol , so it feels more complex and requires more manual setup compared to something like Twitter. Because of that, a lot of people’s friends aren’t here yet. Also, many people choose ease of use over everything else, and Nostr hasn’t had much marketing to get the word out. That’s why it’s still pretty unknown to most.
Nostr is a decentralized social network protocol. There’s no central server or company just users, apps, and relays. Your identity is a cryptographic key instead of an account, and you can use several diffrent apps with the same identity as long as they use the Nostr protocol.