They knew it because our Professional Journalists and lots of academics

have been buying their crap and pushing it for literal generations.  And show no signs of stopping.

Borrowing from Insty,

...For the same reason that Ted Kennedy knew that same media had his back when he was smearing Clarence Thomas during his confirmation hearings/show trial in 1991:

I liked political theater, so I watched that initially rooting for Clarence Thomas to be taken down. By the very end, I believed the seeds of my future were planted right there when I realized that Ted Kennedy knew something. What did he know that he could ask Clarence Thomas these questions of a man who came from humble beginnings as an African-American? What did he know about the media? What did he know about the political process? What did he know about Hollywood? That they would side with a known Lothario with Chappaquiddick, you know, in his background. That they would allow for him and his allies in the Democratic Party to ask this man in the realm of privacy have you ever rented any pornography. Do you know who Long Dong Silver is? It was so obvious that this was a takedown that eventually I started to see things through what I called the Democrat Media Complex.

—Andrew Breitbart, in conversation with Peter Robinson, April, 2009.