It's just a matter of how big the target is. Right now, Android is the largest mobile platform, so that's where the malware is directed. It's a crime of opportunity, no different than Windows on the desktop.
It's not proof that Apple's iOS or MacOS or Windows mobile are intrinsically more secure, but that they're smaller targets. How much malware is there directed to FreeBSD or OpenBSD or vxWorks in comparison? Emphasis on comparison - sure, there's malware directed at anything which might be Internet facing, but the more esoteric stuff is more specifically targeted, like Stuxnet.