The short of it is ... Why would anyone hack Linux or Mac?
Ah the market share argument. By the same token, why would anyone hack Vista? Maybe, and I speculate, its much touted security is a result of low market share? I don't believe that - but surely the argument holds?
Not really. The difference there is that Vista's share is growing by millions a month simply due to the normal rates of the PC industry. It'll be a solid number two behind XP in probably a year.