Microsoft and Viruses

Why does everybody focus on Windows when they write viruses? I have a couple theories.

• Writing a virus for open source is too easy - there's no challenge in finding vulnerabilities. Instead of writing viruses, people use Linux's vulnerabilities to do direct assults on other machines.
• If a person writes a good virus for Windows, it can cripple entire enterprises, businesses and government agencies. Writing a virus for OS X might affect 10 people. Of course it also has the potential of collapsing elementary education, but that's about it.