Basically, this is a successful old-fashioned computer worm, operating at a scale we've not seen for more than 10 years. On modern operating systems most attack surfaces that were easy to crack in the past have been locked down at least to the point where it is nearly impossible to find an exploit in a common protocol like this that doesn't require user interaction (hence the popularity of phishing). Apart from that we've just gotten lucky, really. Many of the most catastrophic bugs in recent years (Heartbleed, etc) were never successfully turned into exploits of this nature. Instead they were discovered and fixed quickly by researchers.

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.