Ask HN: I must be missing something about WannaCry etc.

I understand that the massive WannaCry propagation is thanks to unpatched Windows systems such that, once the malware is inside a given computer, it spreads across the LAN that computer is part of.

But as for the initial "infection", it's caused by merely opening a dodgy email attachment. In which case it's no different than any other crap one gets to their computer by recklessly opening attachments.

So what's so unique about this one that makes it spread so widely?