Because, faced with the fact that C++ makes an octopus by nailing extra legs onto a dog, D decided the right thing to do was to staple-gun them onto a cheetah instead.

D, at the end of the day, is C++ with cleaner syntax. Yes, you've gotten rid of headers and some of the syntactic ambiguities, but things that were mutable are still mutable, templates are still templates, hard-to-solve threading problems are just as hard to solve, etc. Indeed, I'd argue this is one of the reasons why C++ coders generally like D: it's the same damn thing with the most painful dumbth removed.

Rust, on the other hand, focuses on having different semantics from C++, in order to make certain problems much easier to solve. The big one is immutability, which makes code easier to reason about, and vastly simplifies multithreaded code compared to its D or C++ counterparts. But there are other major semantic differences, such as region pointers, the lack of nulls, and more, that really solve the semantic issues that crop up in C++ programs.

I know people who like D, and are highly productive in it, but Rust is solving a very different problem in a very different way.