Blender Foundation, as usual, is full of work and projects. Right now they are deeply busy with Blender 2.8, and as a result it’s very likely that 2.79 will be the last release until that BIG ONE in 2018.
But on the Cycles-side, great progress and cool features appear often here and there. Who can tell from here to an year what new whistles and bells could have been worked on? Adaptive sampling, bevel and AO nodes, multilight, lightlinking, and more… all these stuff seems to be almost at reach, behind the next door. And some of them can actually get in master sooner or later. But, I can’t believe that we are going to wait until the whole 2.8 project will be completed to put our hands on a newer (official) Cycles version.
So - thread title question - since Cycles is an addon, why not release it as such? A simple “Download Cycles ver. X.x” button on the webpage, and maybe in the future the GSOC addon management stuff could come in for this.