The other reason is that because mlua is just a wrapper around the C code, it has unsafe you can't really get around. So for example Lua is used in Redis, which has this critical CVE https://github.com/redis/redis/security/advisories/GHSA-4789... that a memory safe version of Lua wouldn't have to deal with.
Mlua is still fine or even better for many other cases though!
It just seems like a lot of hassle to write a lua interpreter, although it would be nice to see a high quality one in Rust :)
Hematita was promising, but looks abandoned.
And yes it seems like there has been many attempts to get a solid Rust Lua over the years and most never reached parity so hoping some people can find use case for it! This one is at full parity in terms of behavior and performance is getting to within striking distance.