I suspect it's actually the cumulative effect of three factors:

1. Macs are a less attractive target because there are fewer of them, which also makes a virus that depends on lots of connectivity less likely to propagate.

2. For the same reason, there are fewer people who have the detailed familiarity with the platform that writing a virus requires.

3. At the risk of starting another holy war, I suspect that Macintosh users are, as a general tendency of the group and not denying the existence of many exceptions on both sides (is THAT enough to satisfy the objectivity police?) less oriented to the technology inside their computers and hence less likely to be able to write one than are Windows users.

That said, the headline at the top of the cover of the current Popular Mechanics claims to finally settle the PC-vs.-Mac debate. Their conclusion: whichever one you like. Amen.