The issue being exploited in the ransomware attack was patched months ago (MS17-010). The reason that it's successful is a) people don't apply patches quickly

The issue being exploited in the ransomware attack was patched months ago (MS17-010). The reason that it's successful is a) people don't apply patches quickly b) people us unsupported versions of the operating system and c) the NSA dropped a reliable exploit for it. All software has bugs, from all vendors. Security patching is a fact of life.