- If you enforce memory safety to all of your code, the number of the libraries that you can use will be severely limited. Imagine if you can only use C++14 libraries. How many C++14 libraries are out there? And I'm not sure if there will be many libraries written in safe C++, at least for the next few years. The adoption rate for newer C++ standards (C++11, C++14) was generally not satisfactory.
- If you only enforce memory safety to your own code, then you could use the existing ecosystem. But in that case, your program will still segfault because of the libraries you're using. Admittedly this is also a concern of Rust when you use C libraries in Rust. The more safe code you use, the safer your program is. The problem is, if you choose this route, there's not much ecosystem-ish advantage in the C++'s side. You will also have to "reinvent the wheel" as you would have to do in Rust.
This problem all boils to this: adding guaranteed memory safety to the unsafe language is a breaking change. And the problems occurred by breaking changes are very hard to solve.