The main selling point of Node was that it was JavaScript, and since you are going to have to use that on the front-end anyway, you might was well use it on the backend.
I've never seen anyone argue that callbacks are one of the easier ways of doing async.
No, the main selling point is that it's an event-loop based paradigm where all the APIs are async by default. As opposed to python twisted or ruby eventmachine where you have to look for async versions of the libraries you want to use and people writing modules usually aren't even aware that you want to be event-based.
I've never seen anyone argue that callbacks are one of the easier ways of doing async.