First mover. For a long time D targeted the same abstraction level as Go, and most of its libraries were designed with that in mind. Stuff like no_std was never a major goal for most supposed "systems languages" (that are not C/C++) until Rust came along. D was too slow in gaining momentum, getting corporate sponsorship, and adding new features that pushed the boundaries of what a systems programming language was capable of. Go had first class tooling, D felt like a C++ with a Garbage Collector. It reminds me a lot of Ocaml which also had a lot of great ideas and occasionally showed up in the engineering blog of a major corporation but if you look closely then you would notice severe issues with package management and multithreading (both of which are finally getting attention and being fixed, 5 years too late). There are a lot of boring engineering that goes into making a good programming language ecosystem that many smaller languages do not have either the willpower or resources in tackling.

This is not criticism of the D language, programming languages are like startups, success often has little to do with the product itself but more the ecosystem, funding, and luck.