Does not play well with anything else, and the primary langage is Java/Kotlin, even making two “managed” languages interact properly is difficult when both have well behaved ffi.

It would require rearchitecting everything to work with separate processes and IPC, and that’s got some overhead(s).

It also has memory safety issues around concurrency, and its limited allowance for abstractions makes papering over those complicated.

The GC I guess ? Rust is mostly replacing the C & C++ code that is pretty low-level in the OS where a GC is not desirable or even usable.

They already have managed language, why would you add another one?

I'm guessing if you don't need something low-level/c-compatible in Android you'd reach for Java/Kotlin since it's already engrained in the ecosystem.