There is a touch of Microsoft having a special relationship with manufacturers, so that no competing operating system could ever have a chance.
When the first netbook from Acer appeared with a Linux distribution, Microsoft came in and cut the deals.
I think the way it works is that if a manufacturer supports only Windows, then they get special discounts from Microsoft for their OEM Windows. Since most manufacturers still depend on their Windows sales, they cave in and exclude Linux.
These deals are obviously kept secret, and I wonder whether a competition authority should look into them.