Well... mocha has support for callback since the beginning, callback is the simplest way to go, callback will never become obsolete, so there's really no need to use promises. :)
If you call function x with a callback you either handle it immediately via your callback or you have to use some other paradigm to make it compose with other async code. You get no return value allowing you to write code that looks synchronous and forget about the beauty of async/await you've completed abandoned it with callbacks. If you use Promises you can do as you wish and combine them as you wish as they hold values and you can use them multiple times trivially with other Promises,