Other commenters have given most of the reasons already, but since you asked specifically for the author, I'll chime in as well.

The fact that Roto gets compiled at the runtime of the Rust application is very important. That means we can ship a binary and still allow scripting.

We also believe that Rust is too complicated for our use case in some respects, we're trying to make something simpler. Our target audience for Rotonda is not people who necessarily know Rust. We can never be as simple as Lua because of the static typing, but we're trying our best.

And finally, we don't have to ship the entire Rust toolchain with our application. Roto is fully embedded into the binary with no external libraries needed and that's quite nice in practice.