According to Leo Laporte (on the podcast Security Now), Mac users are now targeted by malware because most of the larger base of Windows users use security software.
Here is the exact quote:
LEO: No. And the numbers game is that, as Windows users get more sophisticated, and 80 percent of Windows users now run security software, that 20 percent remaining is roughly equal to the penetration of the Mac in the marketplace. So now the Mac is at parity in terms of insecured users, unsecured users, so of course it's going to attack. I mean, it's actually, you could just watch that number as it went up to 70 percent, which is now roughly equal to 20 percent unsecured Windows users. Okay, I guess it's worth the effort.