The same reason they made sure that the CIA News Network was there to film it. (Though I think their film crew must have been sleepwalking.)
Paul Joseph Watson re-tweeted this. (sad reading some of the replies):
TWEET: Greta Van Susteren: CNN cameras were at the raid of Roger Stone...so FBI obviously tipped off CNN...even if you don’t like Stone, it is curious why Mueller’s office tipped off CNN instead of trying to quietly arrest Stone;quiet arrests are more likely to be safe to the FBI and the person arrested
25 Jan 2019
https://twitter.com/greta/status/1088769013344337920