I quite liked TomP’s answer:
One major difference is that Ada was created at a time when most military computers were single-core with in-order sequential execution and no cache. Although limited SIMD existed, any other concurrency was very coarse-grained. That’s the underlying execution model for C, Pascal, C++, etc.
Rust was created to address the complexity of multi-core processors with multi-level cache hierarchies where computational efficiency may require much concurrency. In my experience few humans are capable of error-free design and implementation of highly-concurrent systems unless they employ tooling that flags their errors in conceptualization or implementation.