Rust brought something new that no other language had: memory safety through statically enforcing object ownership and borrowing. Anyone who has done manual memory management already is familiar with those concepts, and how difficult that can be to track in large systems. Whenever a language is able to abstract pervasive concepts to first-class entities in a language, it becomes attractive to people who deal with those concepts on a regular basis.

D, from what I am aware of, did not offer any new abstractions. Rather, it was an attempt to do all of the existing abstractions in a better way. Which is laudable, but it can be hard to convince people to switch to a language when it offers only iterative improvements.