It's free - for the most part. You can pay for RedHat and some corporations do for support contracts.
it's open source - created by a programmer, for the most part, most of what runs on Linux is open source
It's programming languages are free - back in the day when Borland and Microsoft charged for programming languages (and MS still does), all the tools you need to build your own software for free? Yes, please
It's incredibly customizable - a Linux distro can fit on a 1.44MB floopy disk if need be. It's like running Android on a refrigerator - it can be customized to fit on almost anything.
It can be a TV interface or it can run a multi million dollar website. When your just starting out as a programmer and don't have a lot of money; you can get started quick and easy. When you have plenty of funding and can hire a team of programmers, you can get started quick and easy.
It's generally incredibly more stable then Windows Server
It's generally more scriptable then Windows
Apple stopped making servers years ago so that's out.