There are many useful communities on Stack Exchange besides Stack Overflow, like Ask Ubuntu, Super User, which are used by many talented and expert users (in a particular field). Why don't these communities have a career website and why doesn't Careers provide job opportunities to users of these communities in their respective fields? It would be great if Careers would extend its approach to Ask Ubuntu for Linux Jobs and Super User for System administration jobs.
It's really about focus right now. Developers and SysAdmins are the focus now because as good as Careers is for them, we're still just scratching the surface. Not just in terms of market share, but in terms of our ability to make getting a job a better candidate experience. If we thought our job was done, or at least further along, we'd branch out.
There are jobs on Careers already that would match career paths for users on Super User and Ask Ubuntu. We don't advertise there yet (specific jobs, we do have Careers house ads), but expanding our presence for certain relevant jobs on these sites before we tried to create a whole new silo of jobs is more likely.
Technically it could be expanded to include (all) other sites of course. I think the reason that the Careers site is just for SO, is that there is actually a very large team working for that site (most of the SE employees work on Careers, not on the software).
I guess that, since SO is the largest site and has the largest user base, it is the most interesting site to have this running. Maintaining relations with business partners is a very expensive business, so the return on smaller sites will be less than it is now for SO.