Sometimes it pays to keep a low profile. Navy and Air Force pilots refer to it as flying under the radar, but IT people refer to the concept as security via obscurity. No matter what you call it, the idea is simple: if you don't draw attention to yourself, you should end up with less problems.
In the world of malware this more or less means that it's not worth a developer's time to attack anything but Windows operating systems because of the research/development time versus the potential installed base. Now that Mac OS X is picking up popularity, it seems that it is finally a blip on some radars.
“In some cases, attackers will seek to exploit vulnerabilities such as currently unpatched flaws in Apple's QuickTime multimedia player application. In other cases, malware writers will use threats based more on social engineering, such as with the MacSweeper rogue cleanup tool that appeared during Macworld Expo, the researcher said.”