The main reason the way I see it is that a lot of people who still use C and C++ need maximum speed, and D is only memory-safe with its garbage collector enabled. This affects latency and adds overhead. For many cases using a garbage collector is fine, but D was trying to target video games and other spaces where the GC was not acceptable. So D allows you to turn the GC off, but then it's just a vastly less-mature C++ with similar safety footguns and worse library support. Rust actually fills a space that wasn't filled by anything else - memory safety without a garbage collector. You can write pretty ergonomic code in Rust (similar to how code would look in a dynamic and/or GC'd language) just like D, but where in D you would have to choose between highest-possible efficiency and safety, Rust gives you both at the same time.

This isn't to say that Rust is better than D, but it is to say that Rust targets a gap in the market whereas D is trying to have the same tradeoffs as C++ but be better, which is nowhere near as compelling for most developers who've invested a lot of time into learning C++. C++ developers don't use D for the same reason that most people don't switch from qwerty to dvorak even though it's technically more efficient. It's not such a qualitative change that it's worth the retraining.