My guess (as someone who hasn't used D) is that it looks/feels too much like C for people who want a better Java/C#/etc, and too much like Java for people looking for a better C, resting in the middle, not really being appealing for either use-case.

GC makes it look like you won't do low-level things (e.g. pointer tagging or very-hand-optimized hot loops which might need to temporarily discard any and all safety), and being called a "systems language" makes it look like not very high-level either. Emphasis on "looks like" - not many will get to the point of actually looking for the options (especially when in a more opinionated language you don't even need to, as there either is an obvious way, or there's none).

Rust managed to stay distant enough from Java-likes (mostly due to manual memory management), and Go is pretty distant from C all things considered (but I'm not too sure why it's so popular. Probably more just being backed by Google?)