It’s hard to answer this question in one sentence.
I think the best answer will be – because I like it. Cause it’s just better. Actually, I should ask another question: why the most of people are using M$ operating system? I’m not talking about those which are using it professionally, because the specialistic programs – their daily working tools, don’t have their Linux port. They don’t have choice – fortunatelly I have 🙂
Safety in web is important. Since I’m using Linux (about year), I was unable to catch any virus/spyware etc… Actually, when I was using Windows, I didn’t have so many problems with viruses, but I know a lot of people who had, I was also many times asked by my friends for removing virus from their computer, also very often it turned out that the only solution was to reinstall system.
About reinstalling – for some people the reason for using Linux may be fact that often Windows after few months of using can be unservicable (i wrote ‘often’, not ‘always’ – a lot depends here on user and his/her experience and knowledge about system’s maintenance). You don’t have reinstall Linux regullary to make it work properlly – even after a year of using you can work on it as on freshly-installed system. Moreover, freshly-installed Linux has all codecs, accesorries, utilities, antivirus/firewall, multimedia and office software alredy installed, in brief – it’s ready to work and you don’t have to spend another few hours installing software and codecs to make your system working. (I have to tell that software included may vary and depends on distro, but practically allways there is office software, email program, web browser, internet comunicator and a lot of multimedia programs.)
If it’s about customizing system and adjusting it to user’s preferences, Linux is certain winner. I don’t mean only customizing appearance (Linux offers plenty of graphic themes for your desktop), but also choosing graphical environment you prefere (KDE/GNOME/XFCE/Fluxbox and more), adjusting all existing system’s options, and even (for power-users) makig your own distribution.
Recognising devices. Linux is non-stop developed, every time new versions are issued. So when some new device appears on market, it’s allmost sure that the next version of system will be supporting it. You can also forget about driver-CDs provided with equipment, losing of which forced you to looking for proper drivers in web – Linux during installation recognize and set-up your devices automatically. (In my case, one of reasons for switching to Linux was buying a new camera, which Windows was unable to detect after installing drivers(!). Linux found it immediatly after switching.)
So why should I pay for something, if I can have something better for free?