This is an excellent article by Dennis Prager on why the left has to suppress free speech.
Short answer: because their beliefs can’t withstand open debate and public scrutiny. As Prager puts it, “Leftism is essentially a giant balloon filled with nothing but hot air,” and conservative free speech is the pin that bursts it. For instance, I challenged readers recently to name any big city that the Democrats have run exclusively for any appreciable length of time that isn’t an over-taxed, crime-ridden basket case that people are fleeing. They can’t, so they refuse to allow conservative free speech anywhere they control, including campuses, Hollywood, certain “news” channels and now, social media.
Prager mentions that he invites leftists onto his radio show and is unfailingly polite to those who accept, but very few do. I can relate: I have a standing offer to prominent leftists to come on my show for a civil discussion of issues. The few who have admitted they were treated with more hospitality backstage than they’ve ever experienced on any TV show. Yet hardly any take me up on it.
Right now, the leftists are feeling invincible, and they’re already acting too big for their britches in trying to bully 75 million Americans to shut up. That’s not going to work on Americans. When the inevitable backlash comes, it’s going to be louder than Ted Nugent.