Most average home users use Windows, because they typically go to a retail store and buy a computer, of which 90+% are pre-loaded with Windows by the OEM. Or possibly shop on line, but with the same result.
Mac has about 6% market share IIRC, but it is more expensive. One reason (among others) is that any OEM can pay a licensing fee to MS and install Windows on their machines, but Apple will not license Mac to any other manufacturer (of desktop/laptop -- not getting into the i-whatever debate here). But many Mac users speak highly of it.
The same licensing issue cost Sony's Betamax VCR system to lose out to JVC's VHS, because JVC (formerly Japan Victor Corporation, a subsidiary of RCA) would license VHS to anyone, while Sony kept its grip on Betamax. The quality of a Betamax image was better, but VHS machines dropped in price so rapidly that they won the majority quickly.
One of the early advantages of Mac was said to be better graphics in applications such as CAD/CAM.
There's a self-perpetuating thing, too. Those whose first exposure to a computer is at work or school are more likely to encounter Windows, so when they buy a machine for their home, they buy what they're already familiar with, instead of learning a new OS.
Many, many more apps are written for Windows, though this factor has been mitigated in recent years with the advent of Win emulators for Mac and Linux-based systems.
Installing a Linux-based system is far beyond the ability of the overwhelming majority of purchasers, and beyond the free-time and energy levels of many fairly tech-savvy users. It's for the real hard-core only. Probably about 2% market share altogether. Support may not be as readily available. OEMs provide tech support and warranty for preloaded Windows systems.
Those who make their living in IT, such as Giorgio or
GµårÐïåñ, must of necessity have copies of all popular OSs, possibly run in a VM. For the rest of the
Support Team, who are all part-time volunteers, it may be too expensive and time-consuming to do that for what is an unpaid avocation.
Some people believe that Mac and *nix systems are inherently more secure. This has been disproven. Most exploits are found and written for Windows, because why spend your time attacking the 7% minority instead of the 93% majority? Mac recently had a huge, 250MB security update, as its popularity rose after the flop of Vista. With rising popularity, evildoers will devote more time to the Mac-attack. (ha!)
Also, many bad-guy hackers learned on Windows machines as children or teens (which some still may be), so that's what they know.
Some say that various Linux distros are not responsive to security issues, and have flaws that go unpatched for months. Of course, the same could be said about Windows.
*Personally*, this is what I learned on, it's what I can afford, it enables support for the 90+% of Win users plus the users of other systems whose problems are not OS-specific, which means, about 97% of the user base. Plus friends, family, etc. And having spent much time customizing this OS to my taste, I don't desire to start all over again with a new one. Also, I once asked a knowledgeable and trusted friend how she liked Win 7. Answer: Fine. Question: Is there anything in it that I would actually want to do (as opposed to what MS wants me to do, or thinks I should do), that I cannot do with XP. Answer: No.
Also, not all of us can afford to buy a new computer every few years, and so may be running with CPU and RAM that are much more than adequate for older systems, but not for newer, more bloated systems. ... To MS's credit, after the legal battles over misrepresenting Vista's resource needs, Win 7 is the first OS in MS history to use fewer resources than its immediate predecessor -- and that goes all the way back to the MS-DOS days. So, maybe they learned something.
p. s.: Someone once pointed me to a very funny
song parody targeting Vista, whose footnotes contained some of the things mentioned here about Vista's problems, both legal and sales.