It's like a privacy friendly social network can't beat Facebook.
Unless Google does something extremely stupid people are going to stick to it.
People use Gmail, YouTube, Docs,Google search, Android and all of them are in sync with chrome. It goes hand in hand with your life.
Same is for Apple, they make iPhone they have mail they have macos. Everything sync with safari. Makes life easy.
What does Firefox offer? Only a browser. And privacy is not going make people switch to them.