... only because Charlie Miller is better at weaponizing his exploits than the people who were going after other systems. He is able to package an exploit so that it can be deployed in seconds. The guy is good.
Real world: There has not been one successful self-replicating pice of malware in the wild for nine years of OS X. There have been many attempts, but the best we've seen is supposedly pirated software downloaded from P2P sites that contained malware that required the user's permission to run.
Whatever the reason, be it architecture or some supposed obscurity, the real world fact is that you are safer on a Mac.