It was based off an SMB exploit released in a ShadowBroker's dump; an unreleased exploit thought to have been used by the NSA.
You are correct about this. Patches were released in March, but many seem to have put off security-critical patching.
In fairness to some of the unpatched - the last round of Windows 10 updates refused to install on some machines (well, mine and some others on Twitter), and trapped me in an endless loop of download-install-fail-download. When this happened my landline internet was down, so this was happening over 4G tethering, and burning up $20/day in cellphone data until I just turned off my internet/tethering.
I'm not saying don't patch (you should!), just that even people trying to stay patched and do the right thing can find they're unable to do so.
I hope Microsoft can find a way to earn trust back, this problem is going to get much worse if people do not install security patches ASAP when released.