You don’t have to be a rocket science to figure this one out. The majority of computer users use Windows; hackers go for the majority. Easy as that. News.com has a great article (obvously more in-depth) about this issue
“A class of attacks that allows a user to take control of any PC or server could leave computer systems in corporations and Internet cafes vulnerable to attack, a researcher says.
Dubbed “shatter” attacks by the original discoverer, the class of security hacks uses the Windows messaging system to request that insecure but privileged applications run malicious code. The Windows messaging system is the medium through which applications and the Windows operating system communicate with each other.” [News.com]