You cannot determine the "best language" (for a given project) from a computer science aspect only. In the real world things like availability of programmers, existing code/interfacing with libraries, support of different platforms etc. play a much larger role.
D may be superior to C++, but if you have to build a firmware in one year and you will need 20 developers you cannot use this language since you cannot hire that many experienced developers and the D compiler may not even target your platform.
Even luck and being in the right spot at the right time by coincidence play a large part. Consider Javascript, which was never designed to do all the stuff people use it for nowadays. But for all its flaws it solved people's problems at the right time and therefore got popular. It was the same with C and C++.
D may be superior to C++, but if you have to build a firmware in one year and you will need 20 developers you cannot use this language since you cannot hire that many experienced developers and the D compiler may not even target your platform.
Even luck and being in the right spot at the right time by coincidence play a large part. Consider Javascript, which was never designed to do all the stuff people use it for nowadays. But for all its flaws it solved people's problems at the right time and therefore got popular. It was the same with C and C++.