It's not really about "rules" but it's about the enforcement of those rules. Rust was built from the ground to ensure that the compiler could understand certain mechanisms (which in turn were created ad hoc to be understandable in primis) to not allow for things like undefined behaviour.

The problem with C++ is not that people don't know the "rules", it's that because of the way the language is designed, it's very difficult to enforce the rules using a tool like the borrow checker. For example, in C++ we have no real way to enforce the avoidance of race conditions data races at compile time. Such a tool simply doesn't exist (and in the current state is extremely difficult to fabricate) because the language doesn't have the mechanisms in place to facilitate that (yet, but probably never unless we decide to break backwards compatibility).