What are the main reasons why Unix/Mac does not get viruses?

Great, the ol' "Argument from Market Share", an essentially un-falsifiable hypothesis. I mean, it would take Saruman, Gandalf and Dumbldore to reverse the proportions of machines with Windows vs Macs, vs machines running Linux. That argument basically excuses Windows and the surrounding culture from examination.

Perhaps Windows is the "Dragon King" of malware, and the Linux or Mac experience is more typical. After all, undocumented systems calls, using part of the file name (".exe") as execute permission, magic file names like "AUX", "LP" and "CON", and not documenting the entire list of file types that can execute might really contribute.

Untrue analogies like "The biggest beast in the jungle has the most parasites" don't help either, they're just more of the same excuse for Windows. Does the biggest beast have the most parasites? Maybe the most visible parasites, but I doubt the most. I personally would expect the most ancient clades of animals to have the most parasites: long-lasting clades of animals would have the most time for parasites to evolve. Some recent paleontological work suggests that some disease we see in birds today may have originally evolved as diseases of the bird's theropod dinosaur ancestors. Since Windows preserves at least a measure of backwards compatibility to MS-DOS, maybe it has the most parasites as the most unchanged clade of operating system.