I think there are just two big factors - garbage collection and tools. When it was made garbage collection seemed reasonable, but with modern C++ and rust we n

I think there are just two big factors - garbage collection and tools. When it was made garbage collection seemed reasonable, but with modern C++ and rust we now know that tracking ownership through different scopes solves the same problems very well with none of the downsides of garbage collection. If you look at D, Julia, C#, Java, Ocaml, etc. etc. over and over you see people eventually fighting with the garbage collection over throughput and latency. D to my knowledge has never fully transitioned to garbage collection being a thing of the past. For tools I think D fell into a common trap, which is to just keep working on the language itself. Eventually the lack of debugger, real time syntax checking and auto completion take their toll and something like C++ seems better even with the rough edges of the language itself.