Their presentation [1] contains a few slides with the motivation for this framework (they have existing Thrift services in C++ and Python, and want to start using Rust too) and why actors (answer: they scale better than a naive solution?). Also a slide "why a new framework" when so many already exist for Rust. (Answer: many are dead/unsupported, are too far from Erlang principles, not flexible enough, or use custom runtimes while ractor builds on Tokio.)
[1] https://slawlor.github.io/ractor/assets/rustconf2024_present...