It's a greenfield space no one else is really jumping upon yet. Focus may have turned to on-demand TV, but people still want to watch sports live, and Twitter already has acquired some of those deals as the sport franchises get more comfortable with online distribution. Trump's tweets, the presidential debates broadcast on Twitter and the fact people turn to Twitter during breaking news make it a logical extension to move into news and possibly finance too.
Twitter's modern-day utility seems very low outside of news/sports/politics and the average joe has moved their engagement to more visual platforms like Instagram and Snapchat where it is much easier to create and consume more personal content and updates.
Twitter would also be able to focus their monetization and advertising efforts around a much tighter content and audience niche. Plus consumers are used to paying for some of this premium content, making monetization of a freemium model even easier.