> Despite stated intentions, I think these platforms want to stay small and niche and non mainstream.

I think that's essentially it, whether they are doing it intentionally or not.

Running a social network is like curating a party or conference. Who you do and don't let participate, and who you do and don't promote above others, is essentially the product. And that is a lot of work.

Open protocols don't solve any of these problems, and people who care about open protocols don't tend to be the same people interested in curating giant parties and conferences for a mass audience.