Because you kee thinking in technological terms. Languages have come and gone, those who stayed are either old enough to have a midlife crisis or are supported by at least one big company. For all the merits D has there is no big name willing to push it forward, even though its creator is now working for Facebook and is doing some internal work: Facebook didn't express any interest in showing this off.

It really is a shame, because D has a lot of things that might interest programmers, all it takes is a little more presence.