I believe the problem isn’t the place where journalist can write, but place to get exposure.
Yes, one can write a blog but how many of the netizens read blogs without a social media link? Independent journalist aren’t newspaper nor magazine. When they investigated something and reported it, yet no one reads it, that’s waste of effort. Social media is the perfect place for them to get exposure.
Twitter of course not the only social media in the world. But in reality, we consume less and less textual but visual information. And Twitter is one of the few platform that focused on text, rather images or videos.