Lots of reasons, but a big one is that dependency and build management in C++ is absolutely hellish unless you use stuff like Conan which nobody knows. In Rust, you use Cargo and everyone is happy.
You can always use something as simple as Make for your C++ proj with manually dumping dependencies to some libs folder.
There are lots of things I don't know until I learn how to use them, duh.
Cargo is great, for pure Rust codebases, otherwise it is build.rs or having to learn another build system, and then people aren't that happy any longer.