Xrdb not loading .Xresources automatically

I am using the following in my home.nix:

  xresources.properties = {
    "Sxiv.background" = "#000000";
    "Sxiv.foreground" = "#f8f8f2";

I can see that ~/.Xresources is getting updated properly, but it does not seem that it is being loaded by xrdb. When I run sxiv, it does not use the settings in .Xresources, but if I run xrdb ~/.Xresources, then the settings are used by sxiv.

Why do I have to manually run xrdb? Is there a better way to make changes to .Xresources and have them loaded by xrdb automatically?