For example, I have still not been able to run Sway as a window manager because I can't find how to set it up properly using Home Manager (doing the equivalent with an .xsession and i3 was no problem). The wiki article about it (https://nixos.wiki/wiki/Sway) has a lot of configuration at the system level configuration.nix, but most of that is missing from the ostensibly equivalent Home Manager config.
It seems like a design error that the Home Manager configuration options do not map 1:1 to the NixOS configuration options for users, and I'm guessing that error is caused by a lack of interest in properly configured multi-user systems.
While I cannot speak to the reasons behind the schism, home-manager is better thought of as independent project that builds off of abstractions in nixpkgs, without taking into account what NixOS does.
And no, it is not a design error to have Home Manager separated from nixpkgs, and unlike a child reply suggests, there's no "schism", and the reasons for separation are rather prosaic, as explained in another blog post of mine [2].
[1] https://drakerossman.com/blog/wayland-on-nixos-confusion-con...
[2] https://drakerossman.com/blog/how-to-add-home-manager-to-nix...