Running affected tests as fast as possible and running all of them in a variety of different browsers to make sure your code works everywhere - are two different tasks. Not like they are completely mutually exclusive, but it's hard to run something very fast, as you type your code, if you have to do it in multiple versions of IE/Chrome/FF and perhaps a smartphone/tablet at the same time. Easier to just use different tools at different stages - wallaby when you write code and tests, karma when you need to run them all in many browsers.