What's special about WannaCry that has made this such a widespread thing? I presume there's has been plenty of malware for a while that can propagate itself around a network of unpatched old Windows machines and people have been trying to get users to clicks on emails to infect themselves for years. So why now? What's so special now?
This worm targets older Windows versions that are installed (and use the exploited protocol) in a lot of critical infrastructure, and the worm was hoarded by the NSA all packaged up and ready to deploy (because it can propagate through SMB and therefore would be perfect for a future Stuxnet-like operation). So of course some criminals get their hands on it, and hey look it works. It's an absolutely bonkers story.