Because distributions don't ship Kent's kernel tree, and they're not going to. Distributions like Fedora ship as close to mainline as possible these days because of the pain experienced from shipping a heavily patched kernel in the past. Release cycles are upwards of 3 months for Linus' tree. With that kind of lengthy release cycle, for an experimental codebase which is undergoing rapid stabilization it was the right call: you don't want old code to linger around longer than necessary when they're predominantly bug fixes that successfully pass regression tests. The choice should be with the maintainer.