https://www.cnet.com/news/facebook-explains-why-its-ai-didnt-catch-new-zealand-gunmans-live-stream/

in a blog post Thursday, Guy Rosen, vice president of product management, said that in order for AI to recognize something, it has to be trained on what that is and isn't. For example, you might need thousands of images of nudity or terrorist propaganda to teach the system to identify those things. 

"We will need to provide our systems with large volumes of data of this specific kind of content, something which is difficult as these events are thankfully rare," Rosen said in the post. In addition, he noted that it's a challenge for the system to recognize "visually similar" images that could be harmless.