Languages are not panaceas. Rust and D are both hard languages, as is C++. Selecting one of these only makes sense if the application warrants it. Each have advantages and disadvantages. I don't really think any of them are suited to web development, but claiming that D or Rust would be a more productive web development language than C++ seems equally silly to seriously considering C++ as a web development language.
Because D is very obviously C++ but better. Rust isn't, though. But D, down to the very name, has lots of things that appeal to C++ users. It has advanced metaprogramming, is multiparadigm, it compiles down to machine code, and has very similar syntax to C++.
D has had other influences, but it is written by C++ compiler writers who think they can do better than C++. I say this as someone raised on C++ and who is now trying D. It is a very natural progression to make, if you're inclined to try something new.
Furthermore, Vibe.d is kind of a burgeoning "killer app" for D. D's package manager, for example, was originally made for Vibe.d. This isn't the only way in which Vibe.d has influenced D development. Vibe.d is one of the most popular D libraries. It's the only D library I know that has its own published book,
https://dlang.org/blog/2017/09/13/the-making-of-d-web-develo...