As Tom Nicholas (the British YouTuber) pointed out, a chat app that's used by the vast majority of the population (like WeChat in China, or perhaps in some countries WhatsApp) can add a payments method and be successful, but Twitter has only ever been a niche. An influential niche, popular with journalists and celebrities, but a niche nonetheless. A chat app first needs the massive audience, and to be a normal way to send private messages to people you know in real life, before it can become a successful payments app.

WhatsApp could perhaps do it, but most of the countries where WhatsApp is popular already have decent payments infrastructure.