Essentially, if you're a media outlet, you'll try hard to not piss off the hand that is feeding you, and Chomsky's assertion is that this is how the media manufactures consent (for all the horrible things the government does to defend the interests of those advertisers abroad).
Essentially, if your opinions are significantly different from those of most politicians (on the important topics), you're unlikely to rise up through the ranks in most major institutions.
Not only negative news about some countries don't emerge in the Western society, but also positive news debunking the falsehoods and stereotypes about the "blamed" countries don't show up neither: https://theintercept.com/2019/08/14/trump-iran-worst-lies
A major problem of our global society is that public perception is biased towards the "winners" and we learn the truth, if we are lucky, about their negative impact long after the damage was done. Example: the US invasion on Iraq was based on the false narrative about the weapons of mass destruction. Again, there were some news outlets that were telling the truth, but it was ignored.
Though with the Syrian conflict dying down, I'd expect others like Yemen to get more attention by filling the "war" slot in media.
All this sort of stuff is claimed under Manufacturing Consent. Chomsky's point is often why don't we hear about these other incidents .. because the news isn't balanced and the government is trying to fashion a narrative and they use the media to do this by selectivly choosing what we hear and see.