Nix itself is written in C++, but Nix the language borrows multiple characteristics from Haskell. Nix community and Haskell community are also very comfortable and comingled with each other and Nix is one of the recommended ways to manage Haskell project dependencies.
Haskell and the Nix language are both pure and lazy extensions of lambda calculus, but that's an enormous language space, and Nix's lack of static types makes them pretty distant.
For many Haskellers, I believe the interest stems from how Haskell's "cabal-install" (its closest equivalent to pip, cargo etc) used to be pretty broken. There wasn't universal interest in fixing this, with some insisting that installing dependencies is the job of your distro's package manager, not your language's ecosystem.
Gentoo did an okay job at having coverage of Haskell dependencies, but Nix was the clear leader ever since they could auto-import the hackage database. It's for this sole reason that I moved from Gentoo to Nix.
This pipeline I came through doesn't really exist now, because the Haskell community has endorsed Stackage, and cabal has fixed its core problems by copying the nix solution.
I thought Nix was written in C++?