Hey, I work on Yarn and Jest. Let me respond to why we built Jest: It solves a ton of problems no other test runner solves and it works well at Facebook's scale. We open sourced it more than two years ago but only recently started to promote the project in open source more actively because it is working so well at FB.
If you are curious to find out more about why we built it, please read our blog posts, beginning with one about testing performance and our unique constraints at FB: http://facebook.github.io/jest/blog/2016/03/11/javascript-un...
Finally, test runners tend to get stale. Any large organization will run into trouble with any kind of tooling as companies scale up engineers or codebases. Testing is no different and we aimed to solve those problems for Facebook; getting to open source our solutions is an added bonus and we are excited to work with the JS community to build the best tooling we can possibly build.