(Note: my frame of reference is experience with UNIX-like systems such as Linux and the BSDs, and even MacPorts on OS X.)

  The operating system package manager is supposed to
  provide packages for libraries that are required to
  run the rest of the operating system. Not libraries
  you need for development.

Distro repos do provide many of the packages you need for development because you need to be able to compile all of the C++ applications and libraries in the distro repo, of which there are many.

  Package managers for development allow things such as
  installing multiple versions of a library, creating
  "sandboxes" with specific version

Distro repos often include multiple versions of libraries because not all of the crucial applications and libraries in the repo are upgraded at the same speed.

  The OS package manager isn't doing the same job and has very
  different requirements. Not all libraries (especially ones that
  are usually built statically or header-only) end up in an OS
  package manager. Typically OS package managers don't bundle
  any libraries that are not required by one or more of
  applications in the package repository.

Fair enough, and for this reason a language-specific PM will most likely have more libraries available, so I concede that point. However, I seriously doubt that a single group of people could be better at providing packages for all the major combinations of kernel, distro, and CPU architecture than the collection of distro groups. Just imagine how difficult that would be!

C++ is different because it is very intertwined with the system. For D it makes sense at this point because it doesn't have the existing distro PM support that C++ does. Once crucial parts of the system starts depending on D the situation might change.