While the MSM has always shown bias by omission (their excuse in the past being that they were limited to 22 minutes on the nightly national news broadcast), they still reported unfavorable news throughout the Clinton administration, albeit with the Democrat spin:
What changed was the loss of Al Gore in 2000. "Selected, not elected" became the left's battle cry. Congressional Democrats slow-walked President Bush's Cabinet confirmations.
Then 9/11 happened.
As the country began to rally around President Bush after his megaphone speech at Ground Zero, the MSM decided to actively censor any news that put President Bush in a positive light.
First up was censoring coverage of the Bali bombings in October 2002, because it supported President Bush's contention that Islamic terrorism was real and international.
The so-called "Bush Doctrine" was to engage the Islamic Jihad "over there" as a national security issue rather than wait for it to come here and treat it as a policing issue.
That's when the MSM went into overdrive to censor the news about Islamic Jihad and adopt the Ted Kennedy October 2003 mantra of "week after week after week we were told lie after lie."
And they haven't looked back ever since.
-PJ
Good summary, PJT.