Ergonomic sum-types, ergonomic pattern matching, hygienic macros, the ability to track the lifetime of every object, generics that have nicer (can still get messy) error messages than templates, immutability-by-default, an insanely good package manager, builitin unit and integration testing, iterators that aren't terrible, easier cross-compilation, a smaller standard library surface so everyone on your team isn't using different corners of the language, and a wonderful zero-cost implementation of linear types are just what I can think of off the top of my head. Oh, and you have complete memory safety if you don't use unsafe, and much stronger guarantees of memory safety than C++ even if you do use unsafe. Adding all this to C++ is just adding noise to an immensely bloated system, and it wouldn't be half as good as a new language, for all the simplicity and compat-breaking reasons mentioned above.

Edit: And move and copy semantics that are unarguably better than those of C++.