Mocha is the most popular testing framework because it's highly configurable and has a large ecosystem of support. Jasmine is nearly as popular as Mocha and quite similar but Jasmine includes an assertion library built-in. Personally, I find Mocha to be more configurable than Jasmine, so I personally prefer Mocha over Jasmine.
Ollie-UI uses Mocha because it's popular, mature, flexible, and boasts a large ecosystem of support. There is also a vast array of documentation around Mocha making troubleshooting much easier than some of the other JS testing libraries available.