D had a lot of momentum in the beginning, but the license around the compiler scared away some people, then there was the whole standard library split with phobos and tango (irrelevant now and has been for over a decade), and from my understanding in the beginning (I wasn't around during that time) Walter added features by request as there wasn't a DIP process yet (D improvement proposals), and some people felt it was unfair. These have all been rectified.
There was also this whole D1 vs D2 where some of us (I was involved with Tango) though D1 more of as a 'better C' (not related to actual better C) and D2 more of as a better C++. Personally, I was more interested in D1 direction. D took the D2 route with Alexandrescu onboard at a time. The whole thing then was also lacking support. Even if D was a better language (D1 was, I guess D2 might be as well - haven't dabbled in a decade with it), you had to do everything from scratch. No libraries, abandoned bindings, etc.. instead of doing work with it, you had to do the plumbing first. That's true for most niche languages. Rust was like that as well, but not seems to have a momentum going on for it and is growing / has grown out of that phase D never did. D has been around now what, 15-20 years? How sweet would it be for someone of Walter's caliber to join the ranks of Rust team? Dude is incredible, but D seems to be a dead end ultimately. Sorry to sound so harsh, but it didn't happen on a scale it needs, why would it now or ever?
The problem is that while D is absolutely better than C or C++, it is only 10-20% better, not a step function improvement like Rust is. A language that's only 10-20% better just cannot get enough traction against a deeply entrenched language.
It may not be a step function improvement but the difference is still night and day: Dreading one more line of C++ versus looking forward to many fun lines of D.
I mean... Modules versus header files, 13 times faster compilation, introspection, unittest, etc. etc. etc. Okay, it's more like death by a thousand cuts but once you produce with D, you can't even touch C++ anymore. I am in that state...
D had a lot of momentum in the beginning, but the license around the compiler scared away some people, then there was the whole standard library split with phobos and tango (irrelevant now and has been for over a decade), and from my understanding in the beginning (I wasn't around during that time) Walter added features by request as there wasn't a DIP process yet (D improvement proposals), and some people felt it was unfair. These have all been rectified.