The language is way too similar to both C and C++. In fact, you can get C and C++ code working in D with minimal changes (most annoying part is int meaning different things in both languages), and nowadays I can pull the conversion off on the regular. It's like the only changes that were made to the language are making things more consistent, making things work better. etc. Rust is more like a language that is a marriage between C-style languages and OCaml. Rust is also a "functional first" language with the ability of opting out from the functional part, first when I've heard about Rust it was through this "fancy and new programming paradigm, that promises to fix everything wrong with programming", and the D team wanted to be left out from making const the default mutability.
The GC sounds scary for most people, who want to develop desktop applications. It's rapid fast, will be even faster soon, and there's tricks to either opt-out from it, or to get a @nogc thread going (using that for audio).
Marketing issues. Walter Bright, while a genius compiler developer, is not a good marketing person.
Formerly D was quite infamous for its toxic users, with Walter going infamous for his "let nazis code" line for a while. Allegedly, most of the toxicity moved over to openD, which made some people question why Adam D. Ruppe forked the language.
The whole shenanigans around moving from D1 to D2, also Tango.

[–] 6nk06@sh.itjust.works 7 points 2 days ago

moving from D1 to D2

I was learning a few different languages at the time, and it's a bit fuzzy in my head, but I remember the confusion between the 2 compilers. One was incomplete and/or obsolete, and the other one was in beta or something and you shouldn't use it anyway. I chose nothing and tried something else because it was annoying to watch. Walter, as the BDFL, should have decided on something but it never arrived, and the hype went away.

All I remember is that some Japanese guy wrote a bullet-hell library and game in D, and that was great. But the community was too confusing for me.