Because most systems in the world to windows, and people write viruses for these systems. If the roles are reversed I'm sure he would be virus on Linux
To elaborate on what was said above. It's [b]NOT[/b] true that Linux is immune to viruses. Yes, Windows holds the bigger market share when it comes to operating systems, but there are viruses out there for them both. It's like saying Mac isn't immune to viruses...it's not true, there's just not a big enough market for it. Most people who write viruses and such do so in an attempt to get famous (think: StuxNet).
But, yes, if Linux held the bigger market share, I'm sure it'd be Linux always under the gun. However, with Linux on the rise when it comes to web hosting and such, I do think we'll see a rise in this situation in the coming years.
One of the key reasons LINUX and Mac are less prone to virus infection is that these systems require privileged access to install and run viruses. Malware rarely is able to gain root or privileged access to these devices and so it is more difficult to leverage system vulnerabilities. The idea that the dominance of Windows is a reason for its susceptibility is specious. LINUX install base covers everything from embedded processors to super computers. While Windows is definitely the dominant desktop OS, the increasing prevalence of embedded devices is rapidly making LINUX the most prevalent OS in the world. All without a major increase in susceptibility to malware.
Virus or other malicious software infections always depend on the activities of the computer user. A computer is always vulnerable in many ways, it is always up to you if how you will secure your Operating System as well as your entire computer unit.
The primary reason is that Windows is an Operating System where it has boot files that executes when the system has started. This boot feature of Windows has been taken advantage by viruses to attached themselves running while the system is in its boot process. Linux operating system has not boot files and it could be considered as an application as well.