The "market share myth" is exactly that: a myth. 10+ years ago, when there were far fewer Macs in the world, there were viruses. Now, 10 years later, Macs have a much larger market share and much larger installed base, and the number of Mac OS X viruses in the wild is zero. Apple sells over a million Macs per month and the installed base is around 100 million. Your statement that "not enough people use Macs" is completely false as a reason for the lack of Mac viruses.
Then tell me why not that many mac viruses have been seen?
People only write a virus if they know they can infect a large number of machines. Why write a virus that can infect 10 million machines when you can write one that will infect 100 million? Yes the Mac has a larger user base, but it is still peanuts compared to Windows.