Well, there are many reasons, Linux is a very good development platform, almost all compilers exist and are free. You can get almost any kind of software for free, wordprocessing, image processing, compilers, 3D modeller, CAD and so on.
Windows is nice to use, but everything will cost you money, and Bill's products can be very nasty, they do not care one bit about other operating systems. One good example is Windows 2000, the installation program will happy overwrite the MBR (Master Boot Record) without a word, for you, and if you had any other OS installed, Linux, Beos os Whatever it is gone, sure you can fix it with an update of the boot record but it's not very fun.
Another good reason is support, most people think that Linux does not have any support and Windows has good support, well the truth something else , sure you can get support for your Windows, put up the money and you will get it, maybe even within a week or two....
On a Linux system, if you run into problems, you just post an email to the developer that created the application and most of the time you get help within a day or so. There are also good mailing lists available to help out and it is all free, can it be any better ?
Sure, Linux is not as good as Windows (yet), if you would like to use it as a desktop computer running Word and Excel, but it is getting closer and closer every day. And if you have any interest in developing your own software, then you are much better of with Linux, Linux has StarOffice, WordPerfect and more...
So to sum it all up, the reasons that I prefer Linux is:
It is a great platform for software devlopment.
The support for the applications I have on my Linux is fantastic.
It is FREE, I can install it on any number of computers, give it away and it is all legal.
It is opensorce, there is not a singel company that rules over my computer.
Mikael