Market penetration is definitely a large issue. Along with that is herd immunity. If you are infected with a virus that 9/10 of your friends are not susceptible to, the virus will not spread. If 9/10 of your friends ARE vulnerable...now we're talking.
One thing I will say - it's one thing to poke fun of Windows in a home environment where people have TERRIBLE security practices. I mean really, can you blame the OS if someone clicks on freemoney.jpg.exe.? What's funny is - in a networked environment (read: every business environment), Windows has the best security at its core. Conversely, Unix-based systems have the worst, despite a reputation otherwise.