"Afraid" is the wrong word. As you said, the MSM successfully brandished those who disbelieve Obama's US birth as insane nutcases in the eyes of the general population. Embracing it in spite of this perception relegates yourself to irrelevancy, when Breitbart's mission was to run counter to the MSM.
It's the same reason Rush and Levin don't embrace the story. It's not some grand conspiracy, despite what some FReepers seem to think.
Mike Zullo of Joe Arpaio’s Cold Case Posse has confirmed that the media was threatened out of reporting on this issue, by Soros. Doug Hagman had reported some details regarding this, and Zullo corroborated those claims.
Here’s a post about this, which has links you can look at:
http://www.freerepublic.com/focus/news/2873887/posts?page=62
Look at it this way. If Rush Limbaugh said "the sky is blue" there are about 20 million morons in this country that would cease to believe that the sky is blue for the rest of their lives and they would gouge their eyes out for agreeing with Rush Limbaugh. Rush knows this and on certain subjects he subtly directs people to other sources that won't be rejected out of hand.
To a lesser extent the same phenomena is true for all right-wing pundits and authors. The mere fact that they say something is the proof that it's wrong and shuts the doors in those tiny minds out there to any other explanation. Forever.