Ecosystem isn't that great, and much of it relies on the GC. If you're going to move out of C++, you might as well go all in on a GC language (Java, C#, Go) or use Rust. D's value proposition isn't enough to compete with those languages.
D has a GC and it’s optional. Which should be the best of both worlds in theory.
Also D is older than Go and Rust and only a few months younger than C#. So the question then becomes “why weren’t people using D when your recommended alternatives weren’t an option?” Or “why use the alternatives (when they were new) when D already exists?”
This is only true in the most technical sense: you can easily opt-out of the GC, but you will struggle with the standard library, and probably most third-party libraries too. It's the baseline assumption after all, hence why it's opt-out, not opt-in. There was a DConf talk about the future of Phobos which indicated increased support for @nogc, but this is a ways away, and even then. If you're opting-out of the GC, you are giving up a lot. And honestly, if you really don't want the GC, you may be better off with Zig.
Garbage collection has never been a major issue for most use cases. However, the Phobos vs. Tango and D1 vs. D2 splits severely slowed D’s adoption, causing it to miss the golden window before C++11, Go, and Rust emerged.
