I'm not exactly a huge fan of MS, but at least when MS basically got a royalty for every Dell sold, there was a reasonable certainty that a vast majority of them were going to actually use Windows. It was the only OS pre-installed, meaning that unless the person was a geek (like basically anyone who hangs around HN), they were probably going to use Windows.

If there were two operating systems installed, there's a good chance that a majority of people would still only use Windows, and Dell would be paying a license to Apple that only 10% of their customers actually used.