because doing it for Windows is easier due to more widely known exploits
Because there are insufficient hosts.So I was wondering why there are no virus on the mac.
There's no need to shout like that, especially when you're incorrect. There are one or two badly crafted Trojans kicking about for the Mac but a Trojan isn't a virus, there's a big difference. There are no actual viruses for the Mac in the wild.
The answer is there are, just not many.
AFAIK Mac os is still based on the unix kernal, which was/is very secure.
That's what i was told.