My theory is that Linux is used by experts, usually people with high-paying programming jobs, who can afford to drop a little more money. Macs are more expensive, so they'll also belong to people with higher income. Windows machines are cheap, so they're bought by everyone else, and (on average) they can't afford to drop as much money on the various bundles.

Also, since Windows machines are cheaper, they're more likely to be bought by cheapskates.