C++ didn't break through because the benefits didn't outweigh the drawbacks compared to straight C (meaning you can implement C++ like concepts in straight C, and not have to deal with a lot of baggage C++ brings).
Rust is a whole other kettle of fish. It has more or less been proven by this point that people cannot write memory safe C, even experts in the language are still introducing memory-unsafe bugs in 2018. Therefore Rust is bringing features the C language cannot offer.
Which isn't to say I know that Rust will be "successful." I don't know that. But I do know it is worth TRYING to see if it can be successful, and C++'s lack of success isn't really a counter-argument.
I'm still sad that Microsoft's Singularity OS[0] never got further developed, interesting concept.
[0] https://en.wikipedia.org/wiki/Singularity_(operating_system)