The person creating the virus cares. Affordability of the platform is a major issue when the producers are coming from eastern Europe.

I'm thinking that suggesting that Eastern Europeans care about the cost of an OS license is a really weak argument.

Malware authors care about exactly one number: the ratio of the dollars it takes to put a malware program out in the field over the dollars collected by that malware program. That ratio is good for Windows, and extraordinarily crappy for Linux. If my mom (and all moms like her) switched to Ubuntu, the ratio would get better.