There's not much monetary incentive for malware writers for a market share that's less than 10%. If you can write malware for something that will have a 90% marketshare audience (conficker) or a sub-10% marketshare, which would you spend the time writing?
People certainly go out of their way to dismiss the fact that Mac OS X is a UNIX based system which is top in security in itself and Windows doesn't offer this. Although it's nonsense what you are saying, let just say it's true. What would be the incentive for me to buy a Windows PC? You make it sound like it's a good thing that Windows gets targeted more due to a larger market share. I could care less if I can get a Windows PC with okay specs for cheap, if my security is in jeopardy or if I have to fight tooth and nail to keep my system from getting spyware that will allow someone to steal my passwords and break into my online bank and wipe me dry then I'll pass and spend a little more money on a Mac.