A Mac owner may know the answer to this better than me, but as far as I know the main reason is that there are a lot more viruses written for PCs simply because more people own PCs than Macs. If you're a dick and you want to take down a lot of people with a virus, you write it for PCs so that way you can get more people. The same is true for Internet Explorer -- more people use IE than Firefox, Safari, etc., so IE is a bigger target than those other browsers.
On another note, I always find it a bit odd when people say they don't have any kind of virus protection because they have a Mac. Macs CAN get viruses, it's just a lot less likely. I was just talking to a friend last week who had a virus blocked on his Mac by his virus protector. Granted, it's the first time he's ever seen a virus blocked, but it just illustrates the point that you can still get viruses even if you have a Mac. And in the coming years when Macs become more popular, I'm guessing there will be more and more viruses written for them. They're definitely a lot safer than PCs, but they are by no means infallible.