In no particular order
It is under GPL, an open-source license that protects the authors from commercial abuse of their code by others. Much of the traditional Linux software stack is written with this in mind, which deeply affects the software politics involved in Linux
It works with the most software and on the most hardware. It can run on anything from a smartwatch to a supercomputer. Indeed many supercomputers use Linux.
It is free. Not just free to use and download, but free to learn from, improve upon and code for. The source is open for all to review; as such you can see how complex programs are built, often easily since the most popular projects are well documented, owing to the contributor nature of GPL and opensource.