Maybe because it is a different type of person who would prefer to use a 3d printed gun. She or he isn't seen obtaining a gun, isn't posting about it on social media, or heard firing rounds 'to test it out'. Might not even have a criminal record, history of violence, etcetera. In other words, someone who wouldn't be (easily) flagged as a dangerous individual could be planning and executing a murder. Also - there is no organization necessary to go from idea to planning to execution. There would be no warning until the attempt.