I'm a little confused about this post. Are you asking why your anti-virus software isn;t working in Linux? Or are you asking why a virus isn't working?
In either cases, the answer is similar: It wasn't written for Linux. Windows and Linux work in quite different ways and software written for one system does normally not just work on the other, no matter if it's a word processor, a browser, an anti-virus or even a virus. (Note: There are techniques to make certain programs run on a system it wasn't originally written for.)
But if your question is, why Linux isn't affected by viruses like windows, there's more. You could talk about the reasons to great length, but to simplify one important reason, Linux is designed in a much safer way than Windows is, so it's much more difficult to write viruses for it in the first place.
Greetings,
Nepomuk