Perhaps the question we should be asking about WannaCry is not "why do so many organizations allow unpatched machines to exist on their networks?" but "why doesn't patching work reasonably well most of the time?"
For the past few days, the world of Infosec on Twitter has tried to find as many ways as possible of saying "we told you so".
To be fair, it's true – we did tell you so: for… https://www.virusbulletin.com/blog/2017/may/wannacry-shows-we-need-understand-why-organisations-dont-patch/
Just because it won't be exploited, doesn't mean you shouldn't patch it.
Just because it won't be exploited, doesn't mean you shouldn't patch it. There is a famous story about the rock band Van Halen whose lists of requirements when performing a show… https://www.virusbulletin.com/blog/2015/04/poodle-brown-m-amp-ms-security/