_an answer_ is performance - the necessity of creating copyable/copied messages for inter-actor communication everywhere in the program _can be_ expensive.

that said there are a lot of parts of a lot of programs where a fully inlined and shake optimized async state machine isn't so critical.

it's reasonable to want a mix, to use async which can be heavily compiler optimized for performance sensitive paths, and use higher level abstractions like actors, channels, single threaded tasks, etc for less sensitive areas.