• "C, but safer" on its own is not very enticing. With no other benefits, it's easy not to switch, and instead promise to try harder writing standard C safely, or settle for analysis tools or sandboxes.
• People who use C often have backwards compatibility constraints. Switching to another compiler and a dialect that isn't standard C is a tough sell. You can still find C programmers who think adopting C99 is too radical.
• Programming patterns commonly used in C (rich in pointer juggling, casts, and textual macros) are inherently risky, but if you try to replace them (e.g. with generics, iterators), it stops looking like C anyway.
So "safer C" is unattractive to people who are tied to C implementations or don't want to learn a new language.
But people who do want to learn a new language and use modern tooling, don't want all the unfixable legacy baggage of C.
Rust dodges these problems by not being a "weird C". It's a clean design, with enough features to be attractive on its own, and safety is just a cherry on top.