From the late 40s to the early 60s the real differences between the American left and right were fairly small.
As such, the press could claim objectivity while in reality it reported with a strong bias because said bias was shared through out most of America.
In the late 60s that began to change as the "New" Left took over the old anti-communist, traditionalist Left.
The press joined up with the New Left early, and changed their broadcasting bias to reflect their new ideology.
As this bias drifted ever further from the American norm, people began to notice its Leftist tint.
The press then claimed that just like those before them they were being "objective". This was partially a lie to take some of the heat off, and partially a poor understanding of history of the sort that is so endemic to the Left.
They continue to parrot this line primarily because they need to control the press. WIthout a powerful propaganda machine the Left knows it is in deep trouble in America - as Moore's comments on Americans, for example, indicates.
And they need control of education.