Lots of reasons why it is. I'll give you two.

1) It can't be that replacing 20 C/C++ shared objects with 20 Rust shared objects results in 20 copies of the Rust standard library and other dependencies that those Rust libraries pull in. But, today, that is what happens. For some situations, this is too much of a memory usage regression to be tolerable.

2) If you really have 20 libraries calling into one another using C ABI, then you end up with manual memory management and manual buffer offset management everywhere even if you rewrite the innards in Rust. So long as Rust doesn't have a safe ABI, the upside of a Rust rewrite might be too low in terms of safety/security gained to be worth doing