Changing languages isn't free. Rust isn't enough better than C++ for most developers (or their management) to switch.

Or, if you're more of the skeptical sort, Rust doesn't seem to be enough better. You could argue that C++ developers don't think they write code that creates memory issues very often. So they underestimate the benefits. (And the borrow checker is a different concept, so they may also overestimate the costs of switching.)

The net result, though, is that they perceive the benefits to not be worth the costs.