They were, until the owners of backpage started giving advice to child traffickers about how to continue advertising.
A child trafficker would place an ad featuring an image of child sexual abuse, with wording that gave coded hints that this was a child. "Amber Alert!"
Backpage would strip out that coded language and run the ad, with the image of child sexual abuse.
Sometimes those children would, after they'd been rescued, recognise themselves in the ads and ask backpage to take the ads down. Backpage refused.
I think the short answer is that backpage.com possibly was protected by Section 230, but federal and state prosecutors kept on harassing them with a barrage of new lawsuits and charges until they found a court willing to say it wasn't protected, and the people running it just gave in and pled guilty in the end. With the site shut down and no end in sight they just didn't have the resources to fight it.
e(5) of Section 230 is a specific carve out that the protections don't apply to sex trafficking content. I believe they used that to argue that Backpage wasn't protected by section 230.
A child trafficker would place an ad featuring an image of child sexual abuse, with wording that gave coded hints that this was a child. "Amber Alert!"
Backpage would strip out that coded language and run the ad, with the image of child sexual abuse.
Sometimes those children would, after they'd been rescued, recognise themselves in the ads and ask backpage to take the ads down. Backpage refused.
https://www.justice.gov/file/1050276/download