The system package manager and the language package/dependency managers do a very different task.

The distro package manager delivers applications (like Firefox) and a coherent set of libraries needed to run those applications.

Most distro package managers (except Nix and its kin) don't allow you to install multiple versions of a library, have libs with different compile time options enabled (or they need separate packages for that). Once you need a different version of some library than, say, Firefox does, you're out of luck.

A language package manager by contrast delivers your dependency graph, pinned to certain versions you control, to build your application. It can install many different versions of a lib, possibly even link them in the same application.