Because people treat it like it's an indispensable town square.

This place is chock full of idealists and techies, it's easy to lose track of exactly how much sheer "inertia" and habit shape people's tech choices. And that's assuming there is a choice at all, maybe you either Facebook up or junior isn't going to any activities, nobody knows what's up in the neighborhood, people will either be harder to contact or (partly understandably) look at you like an asocial weirdo, etc

The Twitter crowd is mostly rich people, that's why.

Because Twitter is a very different service to all the bigger ones. The limit on the length of tweets means, that the messages posted have to be to the point, hence they are easy to quote by news of all sorts. That attracts politicans, CEOs and somewhat celbreties to the site, who want to be in the news and it becomes a self enforcing cycle. The eleven larger ones focus either on direct messaging like WhatsApp, Telegram and so forth or have none text media as their dominant form of content like Instagram, YouTube and TikTok.

I don't know anyone who thinks that or ever has. I doubt it's gaining any new users anyway. It just that the people already there are stubborn to leave. Me included.