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 entirely. The main aim of hacking into a system these days is to allow the running of malicious applications as part of a botnet. There is a lower barrier to entry if your mass-mailer (or whatever) is already running on Windows. Particularly given that a vulnerability in Vista is quite likely to also be present in previous versus of Windows.