a. it never had a real corporate backer, until it was too late (and Rust already stole all of its mindshare)
b. It had a garbage collector, which meant that realistically it could not replace C++. Or rather, it could but you basically had two different Dlangs - one with GC and classes, and one without. It was arguably not necessary, because people already had GC languages that were fast enough and worked for their needs, and those who needed speed were better served by C++, which arguably isn't that bad of a language if you know how to use it.
Rust is arguably the first real contender to C++'s reign because it really brings to the table features you'd be a fool to pass on. D was nice, but it was not worth the switch. Eliminating whole classes of bugs instead is.
