You can write games in C or Fortran, so why write games in C++? You can write things in C++, why make Rust? Basic worked, why make Python or Ruby?
Why does it need "features" that make it "superior"? It should be good enough that he didn't want to use C++, so he made a new thing...
As to why Rust, there's actual historical information about why Graydon wanted to make it and why Mozilla decided to fund that work after it was started.
Jon has said the intent was that Jai would allow him to make more games, because C++ held him back so much. So, yeah, it would need to be superior to C++ in at least this way, and by at least enough to justify the effort expended, writing C++, to develop Jai.
And IMO C++ is painful enough to use that nothing needs to be "better", more ergonomic is good enough for huge productivity gains.