Number of deployed systems (Score 1)

I think the answer lies in the number of installed systems running linux at home. Most viruses/worms today seem to recruit zombie machines to carry out larger attacks. The easiest machines to compromise are those installed at the home without firewalls. Nearly all of those machines are Windows based. That being the case, those who are taking advantage of security holes to carry out attacks focus on creating windows based worms/viruses since machines running windows are more numerous and accessible.

I also believe that if the majority of unfirewalled machines were Linux based, we would see more linux security holes exploited via worm/virus. I believe there is evidence to backup this claim in that there is a higher percentage of viruses/worms per security hole on average, written to exploit windows.