Because CNN is liberal. CNN is known to often cast conservatives in a certain light and associating Trump supporters to conservatism. They rarely bring conservatives who are well-informed/logical, thus painting conservatism in a poor light. For example, CNN often brings or highlights emotional people who aren’t the face of conservatism like the Trump supporter Kayleigh McEnany who was unheard of until CNN brought her on. Personally, I believe CNN could shed it’s liberal bias by bringing on more educated conservatives more frequently such as the former RNC chairman Michael Steele.
Thus, it is safe to say that CNN has a liberal bias. Do note that that doesn’t necessarily mean that CNN is owned by liberal people. Owners of media companies often employ strategies to get the highest ratings and biggest outreach based on their demographics. CNN’s demographics are liberals.
Fox News does a similar thing with painting liberals in a certain light.
Quite simply, fruitful and open discussions between conservatives and liberals are bad for TV.
I’d recommend reading Noam Chomsky’s Media Control where he talks about how media companies are controlled by a small minority of wealthy individuals who do anything to control the “herd” (aka us, the people) such as making it seem like they are following a liberal or conservative bias.