No big manufacturer is going to go all-out on Linux: too much risk and uncertainty; big corporations rarely endorse such risk-taking. Instead, they want to sell some Linux computers but still maintain Windows. This gives Microsoft a lot of leverage in the market.
A perfect example would be netbooks--a while back, there were Linux netbooks available and they were threatening to become successful. I think Microsoft first lowered the price of XP and extended support for it and then threatened and cajoled OEMs into stopping selling these netbooks. A cursory Google search revealed, among other things, this[1] article about Microsoft's tactics.
[1]: http://blogs.computerworld.com/microsoft_strikes_back_at_lin...
The other thing is that compared to the obscene amount of marketing behind Windows and Mac OS, Linux is driven almost entirely by word of mouth. Since there is no really heavy backing, Linux is at a disadvantage even ignoring vaguely anti-competitive practices and bullying on the part of the established players.
Edit: Here[2] is another article with more details that explains exactly what I was getting at about Linux netbooks.
[2]: http://www.zdnet.com/blog/open-source/why-the-linux-netbook-...
From the community, for the community
The Open-PC is a PC for everyday use built by the Linux community for the Linux community. We use only free software. Your help will be welcomed, to make our project even better.
(Obviously there's System76 and Zareason and even smaller companies; I wish them wild success and will eventually purchase from them when I stop getting hand-me-downs.)
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The "top 10" list from Linux.com is pretty reasonable, but it's proof that Linux.com is not the home of real journalism -- the list isn't presented as an image gallery. :-/