They had fantastic detection of headless Chrome and curl requests, a thorough IP blacklist, aggressive rate limiting, and possibly some JS stuff.

I was a contractor at a company where one of the divisions did Craiglist scraping as part of the business model and they had a closet full of laptops doing the scraping part of the job - thirdhand what I heard was separate PC's were necessary for the anti-scraping workaround they were using.