If the market positions were reversed, the Mac OS and UNIX would show their vulnerabilities more than they do.
As it is, Crackers concentrate on what they know and are familiar with. Even still, there are Cracker sites which deal with taking down UNIX boxes.
Cracking != viruses. As for concentrating on ``what they know'', I'd wager more crackers have access to an OS that costs $0 (Linux) than they do one that costs as much as a couple of thousand dollars (M$ family of products).
I have three computers in my apartment and cant run Linux on any of them because of hardware support. My server has a Cisco ADSL NIC that has no Linux driver. My primary client has a Nvidia GeForce 3 with no Linux driver. And my laptop has no LCD support.
It would cost me hundreds of dollars to install Linux in my home, although I have a great Linux server at work. EVEN if I actually bought a MS OS Id still be ahead of the cost of Linux. The fact is that computer geeks rarely end up paying for their software. I think the last time I paid for a MS OS was 3.11.
Furthermore, Linux doesnt support a host of programming languages many of which are favorites of crackers, like VB. And, with Linux, forget gaming, which is a pastime of many crackers.
Linux has its strengths and, in certain circumstances, is the perfect OS but, its actual general usefulness and convenience is still far more narrow than Linux zealots like to admit. Much like Macs.