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.
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.