Stackless has a lot to offer, but it does so in a low-level and straightforward manner. Anyone who wishes to make use of it has to implement a lot of the same supporting functionality, in order to get to the stage where they can implement an application on top of it. And then there is the domain knowledge that needs to be factored into this supporting code, which may only be identified as problems arise and are solved.
Sake is a project that aims to be a high-level framework that developers can take, and immediately use to get started building an application. All the standard supporting logic that gets written, has been written. Written within CCP Games, domain knowledge and corner cases learned over ten years of Stackless usage has been taken into account.