Large government and corporate bureaucracies supply ready-made information that lowers the cost of gathering and producing news. In return they become routine sources with privileged access to the gates. Outsiders without that machinery must fight for attention and can be ignored at the gatekeepers' whim.
When most of a news outlet's money comes from advertisers rather than readers, advertisers gain a de facto licensing authority over what gets published. Coverage that drives them away threatens the outlet's survival, so it quietly gets less room.
Complaints, letters, lawsuits and legislative pressure, which the book calls flak, are expensive for media outlets. They can cost advertising money, legal fees or reputation. Powerful groups able to generate flak can make certain stories too costly to run.
A formally free press can still serve the interests of those in power without anyone giving orders. Market pressures, assumptions journalists absorb, and self-censorship narrow coverage to what suits the powerful. On issues tied to national economic and political interests, the outlets end up behaving much like state propaganda agencies.
The same kind of suffering gets very different coverage depending on whose side caused it. Victims of hostile states are treated as worthy of outrage, while victims of friendly states or one's own government are played down. Elections are framed the same way, as legitimizing or meaningless depending on whether the regime is an ally.