Perhaps rich-text format is easier or more universal and can face HR people. But as part of the "for developers, by developers" culture of SE, including its careers service, devs like me should be able to write a posting as easily as we write SO questions and answers.
It's a little easier, but I also think it's good brand and a nice touch. I've written powerpoint presentations in markdown, because what developer wants to stop being a developer, go take a high school "Business and Technology" class, and learn to make a pretty powerpoint? No, we hate that, we'd rather just write in something like markdown and have it work in whatever endpoint application we need. I also think it's really valuable for careers.SE to work very well with that mentality, since it has a "for developers, by developers" brand, and many of the jobs that will happen over careers.SE are aimed at people who enjoy a technology-aware corporate culture.