The short of it is ... Why would anyone hack Linux or Mac?
A few people find vunerabilities in operating systems and software as its their job. The find it, tell the vendor to give them a chance to fix it, then make the vunerability know so people can look out for it.
Unfortunately there are far more people who look for the same issues then exploit them with Trojans and Viruses etc. They want to cause as much damage and mess as they can. Some even get a kick out of the misfortune it causes others. The more people who suffer the better.
Now, if Windows has approx 95% of the market share then its natural people would spend time attacking that system. Why waste their energy on the (relatively) few people who use the lesser common OS's.
If it were the other way around and, say, the Mac had the 95% share of the market then people would set their sights on hacking that OS. You'd hardly hear of a Windows exploit.
Its as simple as that really.