I don't think "no one cared" is a fair characterisation. The paper doesn't support this conclusion, and from experience I'd say that the two difficulties in fixing unsoundness are doing so without breaking existing code (Rust guarantees stability unless the code is unsound, and introducing previously missing checks can cause sound code to stop compiling, which is a problem if it occurs often) and doing so in a reliable way that can be supported long-term. Rust is currently in the progress of stabilizing next-solver and Polonius, both of which fix multiple long-standing bugs and have been worked on for years now due to intrinsic complexity, performance considerations, compatibility issues, etc. It's not that no one cares, it's just that this stuff is hard and takes time.