IMHO a twitt is limited to a 140 string length. I don’t like to be limited. Human being is always looking for eternal things. So conscious or unconsciously I have a perception of losing my freedom.
I can put my words/thoughts in Medium for example. I don’t have that limit here. So why be limited when I can got attention somewhere?
And finally, Twitter as a web page was designed with fixed width. And seems there’s no plan to go responsive. I’d recommend them to look for Ethan Marcotte.