My guess is that Mac's are selling more and more; making them a target for malware. That plus the mindset "Mac's don't get viruses" doesn't help either, at least 75% of Mac owners I've met don't get an antivirus for that reason.
It's not surprising. They go after what's popular and that's Mac right now. Also seeing as people are under the assumption you can't get malware/PUP/etc also makes it an appealing target. Most users would be incapable of handling it. So I agree with g0ggles here.
This does open the door to Linux based OS's too. Seeing as the Mac is built off that source code it could potentially lead to faster development of viruses for Linux.