Python solves this problem with "virtualenv", where the user creates a virtual environment and installs all programs and libraries into that environment. The user can have multiple virtual environments, and it is easy to switch between environments.
virtualenv solves virtual environment as much as pip solves OS package management. Not that it's not useful, but you see all over where you have little fiefdoms that work pretty well for their little ecosystem, but become a mess if you integrate stuff outside (virtualenv, rbenv or pip, npm, gem, rpm, apt or make, ant, scons, jam).
A lot of these systems are just reimplementations tailored to language-X. Nix is pretty unique and clever. For example, you can effectively version control you virtual environment, it's at the OS level, and accommodates major languages and libraries (Python, Perl, Lua, Java, Go, Qt).
Every language has it's equivalent of virtualenv, and that's the problem: it only works for that language's tools and libraries. I don't use Nix, but I do hack on an alternative called GNU Guix sometimes, and I wrote a tool called 'guix environment' which is similar to virtualenv but applicable to any software. Having a generalized solution is much better than one virtualenv tool and package manager per programming language. In Nix the equivalent tool is called nix-shell.
