Odds are, it has to do more with the small number of linux computers out there. The people who write the malware programs are doing it for the money, not for just kicks.
Now that Apple has reached a certain number of computer users, it’s a viable target and is now being attacked.
From what I’ve read, Apple is just as vulnerable, despite denials because they’ve never had to deal with dedicated attacks as Windows has.
It will be interesting, over time, how well Apple deals with it and, if Apple has more success with it.
I believe Apple uses a (U)nix-style core. Unix/Linux tends to be more robust simply by architecture than does Windows. Unix/Linux do not incorporate the graphical shell into the system the way Windows does, and the graphical shell in Windows - especially given the lack of system-level security in older versions of Windows - has proven to be full of target holes for virus-writing vandals. On the other hand, root kits are the real bad boys in the Unix world but not so much (up till now) for Windows. My $.02 worth.