ADHD. If I have a long-form blog post to write end, I get caught up in going back and rephrasing and re-editing and such, and it never gets finished.
Twitter's paragraph-size chunks with minimal editing (if I really have to, I can delete the last post and re-create it) means that I can't get stuck in the process of editing and revising, so I have to keep moving forward.
So it's really the only way I can write. My wife's gone through and rewritten some twitter threads into a blogpost for me, but she hasn't gotten to this one yet.
BTW, You can also use a threadreader/unroller to make a pseudo-blog-post out of it.