This is an exploit that takes advantage of a known vulnerability in Windows. Everyone runs Windows, so nobody puts much effort into exploiting UNIX like OSs like OSX. If they did, the situations would be similar. Windows rules the roost, so it gets targeted.
All OSs can be exploited. The problem is that this really stings the no longer supported legacy OSs like Windows XP, Server 03 etc. Ones MS no longer patches.
A lot of places run legacy hardware. Making a huge technology change such as an operating system at a hospital isn't easy. Cost, compatibility, people who are "tech challenged" and the amount of time and money that it would take to train the employees to make such a change is usually not financially worth it. Unfortunately, it results in situations like this.