Maturity of the language, quantity of developers available, knowledge base ecosystem (this is more than just library availability, as it includes implementations to solve problems resulting from years of practice by thousands of engineers), interoperability with other languages' libraries, to name a few. In some cases Rust's "escape hatch" to get unsafe has more boilerplate than equivalent implementations in C++: when Rust's safety model isn't as important it might not make sense to use it.

Personally, as a person very experienced with C++ I prefer Rust when it's available on the target platform. The low level, high efficiency computations I might do use numerical algorithms that neither Rust nor C++ are very good for, although C++ is slightly better in this case.