The reason for not using async ajax requests is mostly historical when this library was used only for internal functions in DFFS and other solutions. I agree that using async and promise is better, but because many users have started using these functions already in their custom js I cannot simply change them without breaking their code.
It can obviously be done, but if you want to use async functions you should use REST and not these old webservice calls that I still use in SPJS-utility because DFFS is also compatible with SP 2007 and there is no REST API there.
What is it you like to do asynchronously? – if you give me some details I can give you a code snippet to get you started.