
Why there may be unpatched versions around: money
I didn't know this until I was called in to try and help a customer who actually had this infection.
The virus entered via email on a Windows system (as far as I could trace), which still seems to be the most common entry vector of any infection I've come across. It eventually found the VMWare ESXi Linux box which was left unpatched and proceeded to encrypt its contents, so eventually nuking all the VMs it was running and presto - death.
On querying why this box was running an old version of ESXi I was told it was a cost cutting measure by a since then sacked director. It turns out there are apparently two types of licenses for VMWare, with the more expensive one allowing you to update without taking the system down. As they weren't prepared to spring for the more expensive one, the much needed update conflicted with the need to keep making money, and naturally the bonus-generating option won. Repeatedly. Until they got breached so hard there was literally nothing running anymore.
I'm sure the aformentioned idiot will only mention his cost savings in his new job, not the consequences..