In short: the cost of switching CSS2 to use Bikeshed (and the likely fixes / changes needed to Bikeshed to handle producing a multipage spec like CSS2) is not worth the benefits.
@gsnedders has been able to create a branch of CSS2 (see https://github.com/w3c/csswg-drafts/pull/2544) that he can use to regenerate the existing CSS 2.1 REC with 2016 updates.
That is, at this point it is likely easier (less work) to incrementally maintain the tool setup that @gsnedders has setup to build CSS2, than it is to do the work to change CSS2 to use Bikeshed.
My understanding is that @plinss is also able to run the CSS2 build tools, and even do so automatically on the server whenever a commit is made to the /css2/ directory to rebuild the spec accordingly.
PROPOSED: Automate and incrementally maintain existing CSS2 build tools instead of converting to use Bikeshed.