A language is often not improved by adding features. Rust has a relatively clean slate (for now), which makes it a vastly better experience.

While they introduce constructs that significantly increase the safety of C++ written with them, they still have edge cases due to the C++ committee's commitment to backwards compatibility.

So while C++ is certainly improving, Rust has a fundamental advantage in the safety department.

Rust doesn't have C++98, C++ version "C with classes", and so on.

Less flippant answer; the usual arguments are that Rust is more memory safe, and hopefully less complex and with fewer gotchas compared to C++. It's still relatively complex - inherently, due to what it tries to do - but people seem to be of the opinion that it is much less complex than C++.