I believe some countries require vendors to supply the computer with an OS installed; in this case not needing to include a half-assed version of Windows Home Underwhelm Experience can help them cut away those 10% of $300 low-end models. There's a reason many netbooks come with linux and it's not user-friendliness.

Additionally, more and more governments pass laws that require their institutions to use linux or open source, or choose bidders that do. Then you have an incentive for a company which wins such a bid to use linux on umpteens of desktops.

Those are just two incentives. There are probably more.