Their rationale is that JSON is less bug-prone than XML, upon which the other formats are based. Many developers prefer JSON and are more familiar with it. And if reliance on XML was limiting development of RSS and blogging, Simmons (who was the longtime developer of the NetNewsWire RSS reader) and Reece (who recently launched the micro.blog blogging service) wanted to find a way to escape those limitations.
As someone who was wrangled XML-based feeds for a long time, I can see the appeal.
If other developers adopt and build on it, it could big a great new addition to data feeds on the Internet.
Update: Six Colors now has a JSON Feed available here.