Good article by Andrew Klavan in City Journal describing how conservative voices are silenced.
No person of importance on the right seeks to silence anyone on the left. The Left, on the other hand, is broadly committed to ostracizing, blacklisting, and even criminalizing right-wing speech.
So we stay polite and silent.
Nothing scandalizes a leftist like the truth. Point out that women and men are different, that black Americans commit a disproportionate amount of violent crime, that most terrorist acts are committed by Muslims, and the Left leaps to its collective feet in openmouthed shock, like Margaret Dumont after a Groucho Marx wisecrack.