Very similar to why the New York Times publishes a narrow set of opinions. The government doesn’t have to ask NYT to restrict opinions. It’s just that a series of forces have come together such that one does not become an editor at NYT if they’re a militant vegan pacifist. You have to have a certain set of moderate opinions to get in the door. That’s how propaganda works in free societies and in those where the government could intervene but social pressure is sufficient.
https://chomsky.info/consent01/