Wow, you barley knew of him yet you know he had minimal impact. Just because you didn't know of him does not mean the man didn't have impact - liberals hated him - hated him to the point where they'd tell him to his face that they wish he'd drop dead. You need to get out more.
You realize, of course, that my comment makes perfect sense. EVERYONE knows of Rush and Hannity because of their high profile.
The obvious conclusion is that Breitbart simply didn't have that high of a profile. He was in the 2nd or 3rd tier of commentators on the political scene. He's probably in the class of a Cal Thomas or less in terms of recognizability.