I'm guessing that would scare the crap out of the masses. Security by obscurity may not work but if there ever were a glitch or breach it sure does look bad to say "and they knew about the bug because we made our codebase public"
I don't think you could spin it that way. "The popular media can only handle ideas expressible in proto-language, not ideas requiring nested phrase-structure syntax for their exposition." Though maybe a better reason is that there doesn't seem to have been any such spin on big open source issues (such as Heartbleed) of the recent past. Why would they spin it as an open source problem rather than say a government/contractor competence problem (or something else) in the IRS case?