Well for starters, you could pirate any version of windows for free and load it up on practically any hardware. Can't say the same about mac os unless you want to take the painful route of making a hackintosh
And they made a number of legitimate attempts to clamp down on that. First with CD keys, then with online activation, then Windows Genuine Advantage. Of course, they didn't succeed, because the phrase "successful DRM" is an oxymoron.
Their attempt to clamp down on that only really started with Windows XP, when they already had a near-monopoly. Earlier version technically had a CD Key, but I don't think those actually did anything as long as you passed the key along together with the CD.
Linux is designed to run on pretty much anything, OS/2 was pretty pirateabld and even given away for free at one point. I don't really see that pirating the OS made much difference especially since almost everyone got windows for free with their PC.
Windows came to dominate because of Office. You only pirate something you already want to use.