With nix, you can't precisely specify versions of all your tooling. You have to choose a channel, and you get whatever that channel gives you. You can't upgrade one tool without upgrading everything else at the same time to whatever version the channel provides. For many dev workflows, that's not enough control.
That's not strictly true, particularly with flakes. You can easily pin multiple versions of nixpkgs as a flake input. Also it is possible to override the version of a specific derivation