 They are way harder to stop, and that appears to be the case in this situation. From what we know of this 20-year-old, he wasn't really active on a lot of social media. He was very much a loner, very isolated existence, didn't talk to people beyond his parents for the most part. That's way harder to find. You get terrorists together in their super-secret backdoor channels on Telegram and the like. There are actually a lot of spies there.