Why? They are better for serious techies who care a lot about privacy and data control. But the huge majority of people don't value this highly and they come with major downsides. They are harder to implement, they are harder to monetize, they are harder to curate against objectionable content.

This feels like "year of the linux desktop" to me. There are tons of great things about linux desktops for tech people but it never took off for obvious reasons.