As some of the others have noted, part of Mac owners' protection against viruses is their own low population density -- even if you own a Mac, most of the people in your e-mail address book are probably PC users.
(Interestingly, this works exactly the same as real diseases in biological populations: it is generally accepted that disease was a very small part of human life when we were hunter/gatherers but became a major issue soon after the agricultural revolution allowed for the rise of population centers.)
A Mac virus simply would not spread quickly enough to get very far before it was discovered and stopped.
One of my personal half-baked theories has always been that there are different basic personalities between people who write illicit code for Windows versus those who do so for unix.
The Windows virus writers are people who get some form of personal satisfaction from doing harm (like a kid who kicks over someone elses blocks).
Unix hackers, however, got their experience doing things like breaking into secure servers, not to do harm specifically, but to prove that "you can't keep me out." I may be completely off the mark, but it always seemed to me that for unix hackers, it was always more of an intellectual excercise.
While both want to somehow create mayhem, the type of ego-boost that the two types are seeking is different.