Disclaimer: This is my personal opinion as an employee of Stack Exchange. This is not an official company stance.
First, you should go read the great post Joel Spolsky wrote about the history and future of Stack Exchange, Stack Overflow and Stack Overflow Careers Joel wrote it after Stack Exchange closed it latest funding round with Andreessen Horowitz last month and it does a great job summarizing where the company has come from and where its looking to go.
This quote near the end sums up the direction of Stack Overflow Careers:
I believe [Stack Overflow Careers] is, bar-none, the single best job board for developer candidates, which should automatically make it the best place for employers to find developer talent. There’s a LOT more to be done to serve developers here and we’re just getting warmed up.
While designers are increasingly in-demand, developers are all the more in-demand. The Careers team is just getting started in how we can deliver the best online resource for helping developers find jobs that they love. Once we've figured that out, maybe we'll move into other areas.
That said, Careers does offer a number of UX Design, Product Design, and other various design positions. Why? Because while those may not be "developer-centric", they assist and work with developers regularly (typically).