DJ has been on our roadmap for years and we've been working on it for months. We weren't going to settle for the simple solution. We wrote DJ without relying on existing and not inefficient packages such as Ices, Liquidsoap or sc_trans. We built a new audio engine specifically designed for DJ; it's 25x more efficient than the others mentioned above. This allows for us to offer it a very affordable price. DJ is also built on new web technology. For example: it doesn't rely on local storage to stream music from.
We've put a lot of work in DJ and we want it to be perfect. For that to be achieved, we can't make it immediately available to everyone from day 1.
We will have a beta program that will expand over time to gather feedback from our users that can further improve the system before general release.