Increasingly these days on the web this question is being asked, with a wide variety of suggestions as to why more people haven't switched to Linux. So here are my reasons.
As I run the Mac OS I'm not subject to all the problems with Windows, and I don't just mean malware. Yes, I don't currently have a problem with malware but I acknowledge that Mac OS X isn't totally safe, it's just not targeted by malware writers - it is much more secure than Windows but it is still vulnerable. But the best thing about the Mac OS is that we don't have all the Digital Rights Management restrictions built into Windows - we can reinstall OS X whenever, wherever, and how ever many times we like without breaching any useage restrictions. Try doing that with Vista... One exception to this is a user's iTunes library where they can run foul of DRM restrictions with their existing iTunes library if they need to reinstall the system or change computers. I get around this by not using the iTunes store at all - in fact I still run iTunes 4.2 which suits me fine and is probably the best iTunes of all for just playing music! So the main reason to switch - usability - just doesn't apply to me.
Now, the real reason.
I do like the idea of Linux, and some time ago I decided to at least evaluate it on my MDD G4 as Linux has supposedly evolved into a good solution for people like me. So I did my research, and finally settled with Ubuntu, which at the time was at version 6.06 and was the most popular general Linux release. I read all the release notes on the site prior to downloading, then downloaded the required ISOs and burnt the discs exactly as per the instructions. But I was unable to even begin the install to any disk, let alone the external Firewire drive I wanted it on. Why? Well I'm not a computer newbie so it wasn't a lack of familiarity with computer concepts. I went to the Ubuntu support site and after a lot of burrowing around I found the answer, by accident - Ubuntu 6 was incompatible with my dual G4 Power Mac! Why wasn't this stated in the release notes? Well I have no idea why a known problem wasn't clearly stated up front, and this leaves me wary of any future Ubuntu release - how can I know that any Ubuntu release is compatible with my Mac before downloading it and trying it..?
OS X 1, Linux 0.
Not totally discouraged, I did more research and settled this time on Yellow Dog Linux, which is considered the best PowerPC Mac Linux release. I downloaded all 4 ISOs, burnt all 4 discs, and this time I was able to commence an install on my MDD Power Mac. Commence being the operative word... I wanted to install YDL on my Firewire drive, and leave the Mac's firmware untouched as for evaluation purposes I wanted to select YDL to boot rather than have it boot automatically. Now this wasn't a big problem as although YDL does update the firmware it can be reset within OS X using the Startup Disk panel. But then a second problem arose - YDL couldn't install on my Firewire drive due to a known formatting issue that is seen in some circumstances. I could get around this by completely reformatting the Firewire drive but then I'd lose all the stuff I keep on it. I could back it up, but why should I have to do this at all? I declined to proceed...
OS X 2, Linux 0.
Then there are the known Linux issues of hardware compatibility for things like graphics cards, printers etc. Some of these require downloading even more installers, others require editing system files! Despite being an experienced computer user I'm not about to start learning how to modify Unix system files just to get the thing running - as a long term Mac user I'm used to having the Mac OS 'just work' through every update I've ever done (well, 99% of the time, and the other 1% just requires a minor updater download). I can only recall one instance in 15+ years where I needed to mess with a Mac system file, and that was renaming an OS 8.5 extension (Sound Manager) to force it to load earlier in the startup process to fix an audio problem, a process that had no risks at all and could be done safely by almost anyone with little instruction.
OS X 3, Linux 0.
I currently run OS X Panther, which is somewhat out of date now so limits what I can do, but is rock solid and does everything I really need it to at this time. I intend to skip Tiger and go directly to Leopard sometime after it's released, but I'll wait until it gets updated to maybe .3 or .4 so the major bugs are sorted. When I do install Leopard it will be on a separate internal drive (which I've already installed for other reasons), and I won't be running it as my primary system as I have a continuing need for the solid Classic support I currently have in Panther, which will probably be lost with Leopard (if it even supports Classic at all...)
It comes down to user-friendliness, pure and simple. The Mac OS 'just works', but Linux requires a lot of messing around to reach the same point, and I'd still be left with some software that couldn't be replaced so I'd still need the Mac OS. End of story...