Editors like Sublime and TextMate offer convenience but only limited extensibility. On the other end of the spectrum, Emacs and Vim offer extreme flexibility, but they aren't very approachable and can only be customized with special-purpose scripting languages. Atom is a text editor that's modern, approachable, yet hackable to the core—a tool you can customize to do anything but also use productively without ever touching a config file.
Atom is a perfect complement to GitHub's primary mission of building better software by working together. Atom is a long-term investment, and GitHub will continue to support its development with a dedicated team going forward. As Emacs and Vim have demonstrated over the past three decades, if you want to build a thriving, long-lasting community around a text editor, it has to be open source. Thus, it is.