Media are part of the elite...paid off by the politicians, married to politicians etc. Many are power hungry control freaks who gravitated to journalism because they could control people with their own spin. Control and lust for power is what socialism is, so naturally they are all a bunch of leftists.
Once enough of these leftists gained a foothold in the media and press, they systematically drummed out anyone who was not like them.
Just like in the university system, the leftists gained power little by little until they hit the tipping point. Now there are maybe 8% of professors nationwide who are not raging communists. I think we have more commies in America now than in Russia.