As Linus Torvalds developed a simple Unix-kernel for his home PC he decided to make it open source under the GPL-license, because he used GNU-software in making it and wanted to say thanks this way. GNU was building a free Unix-like OS by replacing step by step every program in System V Unix. They were already pretty far, but they still missed a kernel. Linux was that kernel.
This decision had some consequences. As many people already followed the effort of the GNU-project to build an OS, Linux was very interesting. Linux making the kernel GPL resulted in many of them starting to be interested into Linux and many of them started to contribute to the project. Also Linux completed the GNU-system. So finally GNU could release a completely free system - with Linux as the kernel. This lead to a lot of buzz among the free-software-aficionados and started to make Linux popular.