When that private entity achieves a near-monopoly on search, then it becomes the arbiter of what does and does not get heard.
That's concerning enough in itself for free speech, but add in pressure from government to "self-regulate," and you arrive at effective government censorship/suppression. When Mark Zuckerberg, Sundar Pichai and Jack Dorsey are brought before Congress to testify about what they're doing to stop the spread of "fake news," the implicit threat is that if they don't pro-actively do what Congress wants, then Congress will force them to do so (or punish them in other ways).
That's concerning enough in itself for free speech, but add in pressure from government to "self-regulate," and you arrive at effective government censorship/suppression. When Mark Zuckerberg, Sundar Pichai and Jack Dorsey are brought before Congress to testify about what they're doing to stop the spread of "fake news," the implicit threat is that if they don't pro-actively do what Congress wants, then Congress will force them to do so (or punish them in other ways).