Maybe I'm naive but it could simply be because they are evaluating it for internal use. The FBI and CIA don't want to use vulnerable browsers any more than we do.
I'm going to say that's definitely naive. It's a well-known fact that the US government has gradually been placing more and more emphasis on "cyber-warfare" over the last several years. A 0-day vulnerability allowing code execution (in a browser that's popular with tech-savvy users, even) is a very valuable tool in that regard.